Ken,
The whitworth markings may refer to the steel used but Whitworth wasn't allowed by British Law to re-proof, That has to be done in a Proof House. The alterations could have been carried out by anyone, including a back street 'gunsmith'.
If you can borrow the gun, take it to a qualified gunsmith and pay him for an honest opinion, If he O/K's it you've spent slightly more than the asking price. If he says No. Then you've spent a little money to save yourself a lot.
Remember Proof Marks only say the gun has passed proof (dimensions within manufacturing specs and test firing) and that Proof was only really valid at the time it was done, not decades later when parts may have worn so the dimensions are no longer within 'Proof Limits'.
Old guns should never be shot with modern loads. It doesn't matter how good the manufacturers reputation is.
Purdeys should be able to tell you when the shotgun was made and what loads it was proofed for. All you need is a description of the gun and the serial no.
Harry
Harry
