Howard,
I believe the Vincent factory of long ago had a service exchange system and that this probably included frame members. Others may know better? If this included R.F.M. presumably the 'exchange' part would not be stamped as the supplier would have no interest in or reason for it, and maybe the 'exchange' would have another machine's number on it or more likely no number leaving the owner to stamp his own, or not. There may well have become a few parts with duplicate numbers on different machines this way, no fraud intended! Original registration (buff) books contained engine & frames numbers but when I bought in the mid sixties a tired, old by then and very cheap bike to ride to work, numbers had very little or no relevance to the value or how well it ran, simply that engine and U.F.M. matched the book. In a world with a better grasp on values it was what mattered. Many years later I found the R.F.M. had never been stamped, not a sign of it with the paint stripped, so perhaps it is such a part. It had never affected how well the bike runs and will remain as is. Who knows what a future owner will choose to do? (But one could guess)......................
Col.