Like someone said (above) its the wheel size , springs etc., that determines the propstand length required. You can add a bit (you wont ever have to reduce the length) by whatever means you like . Bob Stafford was well aware of the deficincy and had castings produced with about two inches extra on the foot. This could be made more attractive by profiling it nicely in the lathe (after deciding the length you require) The " official" length was 8 7/8" from the pivot centre to the foot . this being the length as drawn by PEI in November 1931.for
The pre "A" Diamond frame models had the same basic dimensions as the "A"s but with a one piece cast iron pivot bracket rather than the later fabricated type. This did not affect the main dimensions , RFM pivot position etc but strangely the early brake pedal / propstand bracket had a SHORTER propstand mounting lug on it! This would have rendered the whole assembly even shorter. The 1931/2 models did tend to be fitted with 19" wheels and probably shorter rear springs (need to check) The early spring boxes and springs were supplied by Druids ( the fork makers ) and the spring boxes were fitted to their " Enclosed Spring " forks . Vincent obviously thought them a good idea and used them on his sprung rear end as well. For the real anoraks amongst you look closely at the springbox damper knobs on the very early models and you will see that they are stamped 1,2,3,4 to graduate the pressure applied . All a bit OTT given how quickly everything wore settled etc.,
Just to further complicate the issue I have p s legs on my 1936 TTC and my 1938 comet that are 9 1/4" so perhaps the factory recognised the problem later on and added a bit. This extra 3/8" seems to work on my machines but I have seen "A"s using up mosrt of the extra 2" provided by the Bob Stafford castings. Probably down to long Springs.!
Chers and Happy New Year
John Kidd