PALUNC, if my memory serves me correctly most English 12 bores are proofed around .729 and at .740 they are considered out of proof. Back boring may remove all the pitting before the Purdey is technically out of proof, and then again it may not.

If you shoot your Hussey very well, I would suggest you hang on to it, or ask the current owner to take care of removing the pitting and measuring the bores to make sure they are still in proof before the sale.