I'll let someone who actually understands British proofmarks wink
http://books.google.com/books?id=lvBk8df8PjUC&pg=PA139&lpg



So a bore between .729" and .739" (I think 18" from the breech?) would be marked '12', and if marked '12' would still be "in proof" if honed up to .739" (I think)