In 1908 the company produced the now famous "A" frame movement and from that time on all genuine Synchronome clocks were given a serial number.
There were some minor variations in the movement of early models, for example early clocks had slightly different arrangements of the latch for the gravity arm and had no buffer to limit the travel of the gravity arm.
Useful date clues:
After 1926 a buffer was fitted to all "A" frame movements.
Up to circa 1930, the A frame was a fairly roughly finished casting and was painted with a black paint, often quite dull, almost matt, in appearance. Later clocks have a better finish to the casting and black crackle finish.
In the spark suppression circuit, wire wound resistors were used up to the 30's and carbon resistors thereafter.(These resistors are hidden behind the "A" frame)
The coil of the electromagnet which resets the gravity arm was wound with green coloured silk insulated wire in early clocks. Later clocks have a winding of enamelled wire finished with a wrapping of a black material. One source suggested that the changeover was circa 1934 but my records show at least some green wound coils up to 1950. No doubt there was a period when coils of both types would have been in stock.
The Mark II movement was introduced around 1962. It strongly
resembled the earlier "A" frame movement but had been modified
for ease of manufacture with most parts now assembled from
stamped brass held together by nuts, bolts and spacers.
A separate series of serial numbers was used for clocks with the new movement.
Shortly after the movement was introduced, around 1965-70, a new broad case was introduced for the Mark II movement in which the movement was offset from the centreline of the case and the glass in the cases was often painted black.
During the 1930s and early 40s the company made castings and
parts for the Synchronome "A" frame movement available to
enthusiasts.
Clocks built from such kits or parts are often found and are frequently finished to a high standard.
They can be distinguished from a factory built Synchronome because they have no serial number on the NRA plate.
Some clocks built from castings and parts supplied by
Synchronome do not have the NRA feature.
A very early model with
pronounced pediment and well shaped bracket base
See later for some information about early movements.
Synchronome started to use the
Architectural style of pediment (a rooftop appearance) in
1900.
At that time they were producing an early form of movement.
Note the carved detail in the top corners of the door. The door is inset into the case, not overlapping the front as in later clocks and though not visible in the picture the bottom of the case is finished as a well shaped bracket.
In 1908 the countwheel escapement came into production using the typical A shaped backplate. the use of serial numbers commenced from the introduction of this model, probably starting from "1" since the remains of clock serial 15 are known.
At this time both the earlier designs and this new A Frame model were offered in cases of Walnut, Mahogany or Oak.
The walnut and mahogany were of the highest quality and impeccably polished. By the nature of the wood, Oak does not accept such a high polish but the quality was of the highest order.
The finish on the movement was reminiscent of that used for scientific instruments, rather than the usual 'clock' finish.
The architectural style case continued in use until circa 1919.
In 1919 the false pediment is found, that is a flat top with a triangle shape mounted on top of the case so that from the front it resembles the architectural style.
The picture shows one of the earliest of the false pediment models with elaborate mouldings to the projecting top and bottom portions of the case and the door inset into the case.
There may well be slightly later models with simplified mouldings, though I understand that Synchronome did not themselves manufacture the cases (or some of the movements?) so the differing types of false pediment may just be variations introduced by subcontractors.
Clearer view of false pediment
I have been told that the use of a false pediment ceased circa
1922 (serial 954 is known to have a false pediment) and it is
possible that the false pediment style is the least common type
of case.
From 1922 to 1948 a flat top style was adopted.
The top and bottom of the cases project and are finished with a shaped moulding.
There are several variations with slightly different mouldings and/or different door latches. Some of these variations are shown in the "Basic design changes" section below.
Synchronome bought ex WW1 16 lb shells
and supplied these as pendulum bobs for several clocks at
this period.
After the war, in about 1948, a case with rounded corners was
introduced.
All the cases in my records have either a piano finish black
case with chrome fittings
or an oak case and brass fittings. This applies also to the
type 4A cases.
This continued in use until the new broadcase for the Mark II movements was developed.
This is a sub-style of type 4, the standard "round corner"
model.
I believe it occurs near the end of the series.
As you can see it has a square dial and a bar across the door below the dial.
When the mark II movement was introduced it was at first
fitted in this type of case
but within a year or two a new case, the (type 6) Broad case
style was introduced.
This is another variant of the type 4 case.
The top of the case extends over the door.
This model was used in Power Stations.
There are also some models having an all metal case, that is,
no glass in the door.
These latter were made for The London Underground.
This was a wider case in a plain style
with the movement offset from the centre line.
It was introduced circa 1964
Here again there are several minor differences in style. Clocks
with no pilot dial are known and the clocks may have plain or
black painted glass to the door.
Note: Mark II movements introduced in 1962 were initially
fitted in a round corner case (style 4A).
As the Synchronome movement was in use for over 70 years it is not surprising that there were changes, what is perhaps surprising is that there were so few alterations.
The most noticeable change was the introduction of the Mark II movement in 1962, but even this unfortunate abandonment of style for utilitarianism had little effect on the movement itself. It was followed by the introduction of a new broader case circa 1964. Throughout the life of the Synchronome minor changes were made, both in case styling and in the movement, and a few of these are detailed below.
I understand that prior to 1908 some of the very early clocks were built in such a way that the wheel was pushed, rather than pulled by the swing of the pendulum, but that this was quite quickly abandoned in favour of the system as we know it with the wheel pulling a tooth forward by the swing of the pendulum.
Several different methods were used in the early clocks to hold up the gravity arm until the correct moment for its release to deliver an impulse to the pendulum. One of these latch systems had a leather lined V notch which prevented overtravel of the gravity lever when it was restored by the electromagnet.
I have not been able to find any evidence of the dates of these latches.
View pictures of some of these latches.
The final form of the gravity arm latch
included an adjustable buffer
fixed to the backplate and this was introduced in 1926.
The highest serial number I have yet heard of with no buffer is 1224
A noticeable change is that from 1930 the "A" frame itself was smoothed off and finished in crackle enamel. Previously the frame had been a fairly roughly finished casting which was painted with a black paint, often quite dull in appearance.
The coil of the electromagnet which resets the gravity arm was wound with green coloured silk insulated wire in early clocks. Later clocks have a winding of enamel insulated wire finished with a wrapping of a black material. The changeover was somewhere between 1934 and 1950. The pictures I have at present show both black and green coils in that period. (Some may be service replacements or rewinds).
In the spark suppression circuit which is beneath the "A" frame, wire wound resistors were used up to the 30's and carbon resistors thereafter.
Pictures of more Synchronome movements are needed to narrow those limits.
Variations in case locks and latches can be noted but further investigation is needed before this helps with dating.
Up to the present, I know of two types of locks and eight types of latch:
1. Single lock 
This was a single lock with the keyhole
in the front of the case, no latches.
The locks are fitted in the sides of the
case.Two keyhole escutcheons in the side of the case are
all that show externally. No latch.
4. Turn latch with "T" shaped
handle, fitted to the front of the case (which has an
inset door) The keep of the latch turns into a cast brass
catch fitted to the edge of the case side. Two latches
fitted to each case, no information about a lock.
5. Another turn latch with "T"
shaped handle, fitted to the front of the case.
No information about a lock.
6. Turn latch with "T" shaped
handle, similar to No.5 above, fitted to the front of the
case.
No information about a lock.
The clamping lever is fitted to the
front of the door, with the keep plate on the side of the
case. A turnbuckle on the side plate is a further
safeguard against accidental opening
This has the clamping lever on
the side of the case.
Hook and pin type latch. The "hook" is
a swiveling arm fitted to the edge of the door which
rotates into a pin fitment on the side of the case. Two
such latches and one centrally placed lock are fitted to
each case. This type was used for the vast majority of
clock cases produced by Synchronome.
This form of fastener was
probably only used on a broad case (style 6). It is a
latch similar to that used on brief cases etc.. The latch
covers all screws when closed and tightens the door
against the felt insert usually fitted to the case edges.
Two latches are fitted to each case. Lock also normally
fitted.
Shown below are four of the many styles used.
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I believe these mouldings were left to the discretion of the various subcontractors who supplied the cases.
Main dial shows seconds, 
The additional unit to provide seconds
pulses 
A Mark II movement
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| Unit mounted top of "A" Frame, Has mercury switch | |
Hit & miss synchroniser mounted on backboard below "A" Frame |
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| Unit below "A" frame. has slaved ratchet
wheel |
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Unit with "flag" indicator |
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| Synchroniser? | |
Mercury switch unit well below "A" frame |
Many pilot movements show a serial number which differs from the main movement.
Synchronome bought in many of the pilot movements and makers probably applied their own serials.
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| Pilot movement most often seen | Similar with wire wound shunt resistor |
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| Movements for separate seconds dial | Similar movement |
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| Pilot movement of different design | Replacement movement, probably Gents, in serial 4807 |
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| Smaller type of pilot movement | Pilot with extra cam and contacts |
