There would appear to be some interchangability of crankshafts, but apart from the very early models the flywheel was different between A50 and A65.
The early flywheels lacked maching to take a timing peg. All pre 1965 took a ball main bearing on drive side.
All 1966 on took a roller bearing on drive side plus a different bush to take a thrust washer on timing side, they also had a cutout for the timing peg, and the later ones had a cylindrical timing peg.
Looking at the master parts price list from late 1970 BSA listed the OIF crank as a replacement for the 1966 on crank when stocks of the original were exhausted, but with the appropriate flywheel A50 or A65 and to match the timing peg I presume.
If it was me I would not put a pre 1966 crank in a post 1966 case and I would try to match the crank to the date of my cases.