Wild guessing? Receiver body looks like a Maltby, I think - high "shelf" more or less through the ejector screw. FTR may have converted it to a Mk 1/2 to account for the Mark 2 style forearm at the rear? A No. 5 flash hider installed on a No. 4 barrel?? Does it have the markings on rear sight for the No. 4 or the No. 5? They were different. Had read the "waisted" style bottom metal was used on the No. 5's, but have seen on many No. 4 sporters as well. I think there are "lightening cuts" missing from the receiver body, to be a "real" No. 5? Are there scallops taken from the chamber area of the barrel, under the hand guard? Maybe a "normal" No. 4 long fore-stock that has been sawed off?