The following information provided by courtesy of David Turner who states that it is basically a summary of the info located in the book: "Forty Years at Sangamo" by Robert C Lamphier copyright 1939.
The content of that book and more info can be found in: "Sangamo: A History of Fifty Years" copyright 1949. It contains the previous book plus "Sangamo in Peace and War" by Benjamin P. Thomas.
Quote...Sangamo started in 1924 as part of the Illinois Watch Co. The Sangamo clocks were unusual as a small motor wound a spring that ran a jeweled platform. The clocks had a brake arm that prevented the spring from overwinding. If the power failed then the spring would keep the clock running for up to 2 days. The motors were made to operate in a stalled condition and would not overheat. They were very well made.
Production started in 1924 and they were made under the Sangamo name using Illinois platforms until 1929. In 1929 Hamilton bought out Illinois. The clocks were then made under the Hamilton-Sangamo name with Hamilton platforms.
The Great Depression hit the Company hard and they could no longer afford to make such high quality clocks. They made a small non-self starting synchronous clock until about 1934. General Time bought out Hamilton-Sangamo and production stopped. Sangamo continued to make electric meters, transformers, capacitors and other precision radio parts until bought out by Schlumberger around 1975...end of quote.
The companies activities in the UK were under the name Sangamo-Weston. I believe that despite the statement above that sangamo synchronous motors were not self starting, the company is best known in the UK for a small and very well made self starting motor. As always I am open to correction and welcome correspondence.