have a look at the image it shows a broad arrow and some inspection marks now allowing for artistic interpertation
I still think it is British Military the broad arrow is not an inspection mark it is an acceptence mark and there is reference to a W for Wilkinson the 75 is just a inspecters number
I may be wrong but there were shotguns issued to RAAF forces for teaching the basics of shooting in front of a moving target (air gunners) I can't see why the RAF didn't do the same
maybe the home guard???
