Well, I've figured it out thanks to a post I found about a similarly marked Webley. From what I read, the markings aren't for non-English guns sold in England, but rather any gun sold by the British government as surplus.
On my gun the three markings between the SN and 9 M/M are actually one light mark. It should be an arm holding a sword over the letters NP, which stands for nitro proof. This stamp is the commercial proof put on when the gun was sold by a government store. This particular stamp was used in the London proofing house. 9 M/M is obviously the caliber. The mark .752" is the chamber length (case length). I guessed that was the case since .752" roughly equals 19mm. The 13 tons is the cartridge mean working pressure (not the proof pressure). Wiki says the working pressure of 9x19mm is ~40,000 psi. Math would probably tell us that that equals 13 tons.
I'm so glad Al Gore invented the internet.
Interesting link:
www.riflebuilders.com/proof1.pdf