In 1877 the Harder patent ( now under the control of deGruyter ) lapsed thus allowing other clock-makers to commence production. Many did and copied the Jahresuhrenfabrik clock exactly.
Gustav Becker Co. took their time and commenced production of a Harder patent 400-day clock in 1902. Ever since they started clock production GB had put serial numbers on their clocks and this continued with the 400-day clocks. The serial numbers that were applied to the earliest production 400-day clocks are around 1,632,650
Every clock made by GB had a serial number stamped onto the back plate. The following is a year / serial number list for dating GB clocks. Considering that Gustav Becker was involved with the early development and prototypes the list starts at 1880.
1927 Restart at 0001 due to take-over by Junghans.
GB commenced production with a disc pendulum and introduced a 4-ball design in 1915, however GB was the only company to supply disc pendulums after WW1, in fact they never stopped supplying disc pendulums ! An excellent review of the GB disc pendulums is in the Torsion Times Vol. V, No.2 , pp 42 – 50.