These are generally regarded as one of the most accurate systems, and thus were normally built to a very high standard. Although having said that the factory did also supply these clocks in a kit for the home engineer to assemble himself. A poorly made kit version will run, but the resultant finish, and certainly the accuracy, of some of these kits can be a long way below the genuine Synchronome standard. So that cheap Synchronome on Ebay may not be such a good buy - always check for the serial number as a starter.
All the factory built models should have a unique serial number on the Advance / Retard plate, something that a lot of clocks on Ebay do not have. They are sold as Synchronomes, which indeed they are, but they were not built by the factory!
The genuine models built by the Synchronome company are able (to quote from their booklet)"to keep time to within 2 seconds per week, and often much better"
This is all helped by having a pendulum rod made from Invar, a metal that shows far less expansion due to temperature changes than does steel.
The best you can hope to achieve is around 2 to 3 seconds per month if the clock is in good condition and is set-up correctly in the right environment.
The operation of these clocks is very simple, like all good inventions are, and only uses one wheel, and less than half a dozen moving parts. The secret is in the quality and how these parts are assembled
'Power Station Version' Page is HERE
For a detailed explanation of their operation go to Page 2