Maybe we should have a ‘sticky” thread on how to spot incorrect frame numbers, and how to date a frame and engine as it does come up a fair bit.
It seems there are many reasons why a number was not actually stamped by BSA, some legit (new frame or case fitted) others obviously not so legit
I know some like to keep the numbers “secret” but engine numbers are generally obvious for all to see, and as far as I can tell frames have the same stamping “font” .......frame numbers are less visible.....but here is a selection of engine numbers from this era for comparison (and two from a bit earlier).
A “feature” not so easily replicated is that in some years the BSA letters and numbers seem to be different heights!
Edit: I’ve no reason to think any of these numbers are not legit, but you never know eh.