Hi Bruce, Your bike doesn't appear in the BSA frame or Engine ledgers, and it's not listed in the postwar keycards. And it doesn't fall into any known contract.
But I think that makes it all the more exciting. Have you got a frame or a complete bike, if so what is the engine number?
And does it have sidecar lugs on the frame, and is there a brazed on lug for the prop stand.
There is also a vague possability that the number has been restamped, can you take a picture of the number.