Oh, just when I thought I have all possible wrench sizes now.
Actually I think this BSA was originally destined for the Italian market from the factory. I "think" British manufacturers shipped the bikes in parts and then were assembled by the dealer at the destination?
Because I see some of the screws are marked "USAG" (Italian?) and some are marked "Zimmer" (German?). They are also marked 8.8 (metric grade) and with the three radial lines that mark the SAE grade 5. They are used on frame coponents but not on the engine. They seem to have metric sized heads.
The threads are weird. I have recently bought a tap and die BSF set too. I can thread the old screws onto it (the 1/4" screw die)
I see BSF has a 55 degree angle thread. I think the UNF and UNC threads in the similar size are too different to work and would completely strip out the BSF screw?
But reading further, there is also the British Cycle Thread that has that same pitch but a 60 degree angle. That one would work very closely. Is it unlikely BSA used the British Cycle Thread on the engine screws?