This BSA on EBay; 122100302419 is stated as a '52 model. I sent the seller a message suggesting that it might be a later machine, maybe '62. This is his reply:
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Thanks for your message.
The bike was first registered in March 52.
It's a Golden Flash frame number corresponding with a 52 Plunger frame.
It is still on the original frame, the numbers are clearly visible on the headstock and have not been messed with, but at some time in it's life I'd hazard a guess that it has had the frame professionally modified to make it a swinging arm frame. The frame numbers are clearly original and not doctored.
The engine number corresponds with a 1955 Golden Flash.
It's a nice bike, probably not for somebody who wants original concourse, but a nice old BSA nevertheless.