When I did it I used the BSA owners club the certificate they sent me confirmed that the engine and frame were correct and they also told me when it left the factory which dealer it was dispatched to the date of first registration and also what colour it was.
The person I bought my SA from proudly handed me a BSAOC dating certificate at the sale. It states that; "frame number xxx, engine number xxx, BSA A10 Golden Flash, despatched to xxxx is fitted with an engine of the same period"...
At the time this looked like a nice bonus but I didn't pay much attention to it as I was just looking for a nice old bike to drive around with.
It was only after a couple of weeks I realised that this sentence just meant that it was a BSA A10 engine in a BSA A10 frame and nothing more.
As I said, to me this is not of much importance, but to someone who wants his bike to be as original as the day it left the factory, and in the process of buying from a seller who is waving a dating certificate obtained through the BSAOC then read it carefully before buying.