It's a Mk2 gas cylinder made by Climax Rock Drill at Redruth in Cornwall and still in business too. It's a press fit in the gun and notorious(?) for working loose. You'll see a single row of gas vent holes. Inglis commenced production of the Mk2 gas cylinder but carried on with the double row of gas vents. Why, nobody seems to know but I underestand Inglis changed to the far superior single row shortly afterwards, in line with Monotype - and the drawings!