No 4 Receiver question

[Surpmil]

Personally I'd see that flat on the left wall as possibly a control surface created for either locating the receivers in a jig or jigs, or a surface created for clamping purposes, presumably in a jig.

There was never much enthusiasm for sniping or scoped rifles IMHO from the low priority apparently accorded them, even after the loss of perhaps thousands of No.3 snipers in France in 1940.

After all, if it was a flat for scope mount purposes it was a failure as RSAF(E) and H&H both had to mill off the mating surface, aka "seat" for the front pads.