So many proof marks on the barrel! Very interesting.

The "St.m.G. indicates it was proofed in Germany with a "Stahlmantel Geschoss", or steel jacketed bullet.
I can't quite make out the insignia over the "NP", but the NP is an Austrian Nitro Proof mark. Similarly, the "Pentagon" over the N next to St.m.G is a German nitro proof mark... but I think that's a military symbol, I'm not sure.

Below that, we have the "arm" over NP, which is a London nitro proof mark, and I believe the "V" and "GP" are also London marks. I'm expecting those marks to be enclosed in a circle, with the words "Not English Make", but I'm not sure when they began including that text, which would give an idea when that barrel was proofed. 'm not sure how those numbers represent a date. Perhaps the 94th weapon proofed in 1925?

It has also been proofed for 45 grains of cordite.

The receiver also has an Austrian nitro proof mark.

M1910 on the barrel tells me 9.5X57, but I'd have the chamber cast just to be sure. How long is the barrel?