Yes, I would say the beginning of 1938, although the 38 partslist still shows the 8 bolt cylinder....
I think creating the airgap behind the valvechest was the main reason, that extra ballbearing followed from this to fill up the space
It also meant that the famous 'kink' in the frame around the oilpump was changed from a bend tubing to the casting we all know in order to make space for the wider engine.
This only for the sidevalves as the prewar OHV's retained the narrow engine and bend tubing.