The January 1928 Parts Price List shows a 2-piece shaft, so I doubt that the one-piece shaft is an authentic Model "A" design. The original parts are A-12178 Distributor Cam Shaft & A-12249 Distributor Coupling Shaft. The same part numbers are in the July 1 1931 Parts Price List. The December 1932 Parts Price List has the A-12178 Shaft listed for 1928- 1931, and a B-12249 (was A-12249) for 1928 - 1932. The B-12249 is the only shaft listed for 1932. Could it be that the B-12249 Shaft is the 1-piece shaft?
I have always used the 2-piece shafts.