This re-proof business can get confusing. It all depends on when it was done. If a .500 3" was reproved to Nitro as the full nitro version became common, there will two full sets of marks. Nine times out of ten, there will NOT be a re-proof mark present. Not until relatively recently (1954 rules) has use of the reproof mark become fairly reliable.

You actually have to be MORE careful with later re-proofs. In the old days, the new marks were stamped right alongside the old so that what was done (or changed, if anything) was obvious, and the re-proof mark was therefore redundant. I believe it was actually illegal to remove the old marks. In more recent years, the old marks are actually filed off. (Yeah, you file off enough of the flats to get rid of the old marks, you've got to refit the barrels to the action, which is expensive. Cyril Adams, former owner/managing director of Atkin, Grant & Lang, really went on a rant when I asked him about this.) This means you can't tell what was originally there. Worse, use of the re-proof mark is still weird. My understanding is that if the caliber didn't change, they use it. If rifle was rechambered, it's a new caliber and they don't use it.

Was looking at a nice .375 Flanged Magnum the other day, which was an obvious London re-proof ("18 tons" on a 1920's gun). Old marks filed off. No re-proof mark. Is this a .375 2.5" converted to a Flanged Magnum, or a simple re-proof of an original Flanged Magnum for some routine reason? Impossible to tell.
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