>>64046801
Proofmark means they prooftested the gun with a strong overcharge or two, to make sure that it holds up for even that.

Typically this was mandated by laws in European countries, and how it was done (don't know if it's still done), is that any weapon for sale was sent to an independent proofing house, where they would prooftest the gun and then stamp it.
Countries like Britain would also mandate that any gun imported to the country had to be proofed, even if it was already proofed elsewhere, so on some antique guns which ended up in Britain, or once were in Britain, you may find that additional British proofmark along the original proofmark from its country of origin.

In the U.S of A, proofing was never legally mandated in the same way, but just about every single manufacturer just did it themselves in-house anyway. Any Colt, Ruger, Smith & Wesson, etc, gets proofed at their factory, because A) nobody wants any undue liabilities, and B), it's just a good practice in general from both a societal perspective and from a commercial perspective.

If you make a new gun and it actually fails the proofing, boy, that means there's probably something wrong somewhere on the production line and you gotta go look that up and fix it. Guns doing wrong shit like exploding is pretty bad for business, that WILL catch up to you (see the Sig P320, though that's not for explosive reasons).

>>64046848
American guns were usually proofed even back then because people would ream your asshole if you didn't and your handbomb took their fingers, which they would do either legally or through frontier justice.
Remember, these are places where they would hang you for stealing cattle (as well they fucking should).