Looking again, it looks like a Marine 2/Lt to me. The sound moderator looks like one of the LIST (yes, that's the man again.....) types that were trialled occasionally when required against a GSR. They had really tough steel wrapped casings to absorb the pressures and also had a little (adjustable?) high pressure gas vent at the rear - that you can see just to the front of the tent pole at the rear of the moderator.