Surprising that it survived naval service in that good a condition. Someone must have taken care of it; perhaps it stayed in the lockup as a result.
"Fish belly mag" needed, they no doubt put the standard one in when taken into service.
The two holes in the left wall look old?
The bore is pretty well rounded and worn - must have had a fair few rounds through it and gauges at .306. I’d like to find a ‘fish belly mag’, but they seem unobtainium. The tapped holes do look pretty old - not sure if it would have been fitted with a scope in service.