SF.. Looks a lucky find. The rear frame column supports "gussets" are the same each side and will have the identical forging numbers possibly still visible under the paint and dirt, below the footrest holes. Pre-'53 frames don't have the lug for a prop stand. Late frames have a plate on the headstock for the steering lock pin. The rear axle lugs differ in the position of the grease nipples. Late point backwards, early point out. Otherwise the plunger parts are the same on A,B,M models.
Apart from a basic straightness check, (the top tube sometimes bows behind the headstock on sidecar bikes) head race condition etc, the other things to check are the stand pivot mount holes and the flat part of the footrest lug under the saddle tube. This is where the stand stops often wear away a hollow, so stand then goes further over centre and then lies too low. Easy to rectify now, not so when the bike is all back together and its a hernia lift to get it off the stand. The stand stops also wear, but can be built up.
As you have found it is easy to mount the cases and gearbox. Not so to load a completely dressed unit. It is far easier to assemble crank, cases and gearbox into the frame and rebuild the motor on this stable footing.
Swarfy.