British 4.2-inch Heavy Mortar

[horsa]

That mounting in the center is one I borrowed for comparison to my others in the shot so I don't know it's history. But it is marked 4.2 and has the Australian D/|\D markings on it with a 1943 date. I'd say the one you posted is virtually identical though. Could be yours was an earlier produced 3" mount that was modified to the 4.2" format as those bigger mortars were coming on-line since it has a 1942 date.

For the barrel, the length of the smooth bore pipe is 60 inches with a muzzle outside diameter of 4.75 inches. The first 18 inches from the breech end has an outside diameter of 5 inches.