The engine is a 918 side valve. It has a casting for an oil filter which dates it from 1950 on. I would need to see the front or other side of the engine to determine if it is a USHM2 or 3. The cylinder head however is the same as a WD generator engine and the position of the oil filler pipe in the timing chain cover supports this.
The brass plate on the flywheel housing is a military service plate which would give the regrind sizes on the crank. Military engines were not rebored, they were always re sleeved back to standard bore, this simplified spares stocks.
The side position of the sump drain plug is also to be found on a generator engine.
For more info go to
http://seriesmm.mmoc.org.uk Tech Info WD Generator Engines
