Starting around 1908, when the "A" frame model was introduced, the Synchronome Company marked all production of this type of clock with a serial number (perhaps with one or two early exceptions) and as there was a clock with the number "15" it is assumed that the first number in the series was "1".
It is unfortunate for collectors that they did not date individual clocks, nor is any cross reference list known which would accurately correlate a serial number and the date of manufacture.
In a very few cases there is external evidence of the date of supply, such as an original invoice, so it is possible to interpolate and so obtain an approximate date from a serial number. Information on accurately dated clocks is so rare, however, that the interpolated data can also vary quite a bit and much depends on the assumptions made by the interpolator!
The first clocks produced by the Synchronome Company were of differing styles styles and movements and had no serial numbers.
Serial numbers were introduced cira 1908 when the now famous "A" frame design was introduced, and from that time on (perhaps with one or two early exceptions) all genuine synchronomes were given a serial number.
The serial number was stamped on the Advance /Retard plate (commonly refered to as the NRA plate) which is fitted at the left side of the "A"frame. A few early clocks had the serial on a Synchronome label.
A very few Synchronomes had the number on the armature bridge (Always in cases where no NRA plate is fitted such as when used with a synchroniser).
Numbering is thought to have started from "1" because the remains of of clock serial "15" are known.
During the life of the company, ie up to around 1982, several different series of numbers were used, but two series were predominant.
The first series ran from 1908, when Synchronome started to put serial numbers on their clocks, until about 1962. I do not know the highest number reached, but 6134 is thought to be one of the highest.
When the Mark II movement was introduced in 1962 a new range of serial numbers was started for this movement.
This series probably started from "1" as serial B15 is known.
The highest number I have heard of is 5850.
The first mark II movements were fitted in style 4A cases (round corner cases with bar under dial) as the new broad case (style 6) was not introduced until circa 1964.
This is a separate series which uses the letter 'C ' in addition to a number.The probable dates for such clocks are between 1928 and 1939.
These clocks are believed to have been made for Synchronome by a maker in Coventry, and the Letter"C" may precede the number or be above it.
Occasionally another letter is used with a number. For example, I have a picture of a clock with the NRA plate marked FP 340.