You can be rest assured that if the marks are military, then they ain't 'military' as in the Commonwealth. We had a strict format for our butt markings along the lines (from a Bren Gun......)
KSLI (Kings Shropshire Light Infantry. There was also a SHROP for a yeomanry)
2 - 2nd Battalion
D - D Company (this was usually the Support Company)
6 - this is unknown but it could be the 6th gun or part of 6 platoon

There is a good little book available, I believe from BDLicon, giving the details of the butt marking disc procedures and info right up until the practice ended in 1943 or so