I recently acquired a vent tube for a British 5.5" Field Gun.
It is based on a .303 case but is cut off to 1.765" length to make a straight-sided case. It also has a brass strengthening insert held in at .50" from the case head by three stab crimps.
The headstamp is, at 12:00: MK.5.R.M L 56.
I am unsure as to exactly what these markings mean. Is “M L” one of the Australian factories?
Thanks for any info.