All good Grant, you are welcome. The part does show up as an assembly.
I have now checked Chilwell, for the UK production carriers, I did not find it in the stowage diagrams.
This assembly attaches to the Canadian engine cover protection plate, occasionally referred to as the radio operator protection plate. It is not found in/on the MK-II* rear compartment long side plates, where other supports are located to mount rifle, BREN and Thompson.
Somone made multiple copies of these supports some years back.
Perhaps there are samples with part numbers and maker marks available for comparison purposes?.
Consider reading these threads:
http://mapleleafup.net/forums/showpo...05&postcount=1
http://mapleleafup.net/forums/showth...ght=sten+mount