Stock sub-inspector's mark. Various numbers are known - had nothing to do with the odd receiver.

FWIW, my paperweight suggestion is not destructive, nor irreversible should other facts emerge over time. Nor would it 'lose' anything, because - in my opinion - there is nothing to be "lost". Look upon it more as righting a wrong. Is it "interesting"? Yes. Should it be "legitimatized" as part of a rifle? No.

One of the key features of military arms is interchangeable parts. That receiver would be useless in the field, since it could not be put into another stock. NO final inspector at SA would ever have passed that as part of a finished rifle.

As has been mentioned, there is also the royalty issue - that is why it was not numbered. As for being an "experimental", the trigger design had been long settled at that point, plus, the work is not "finished" appearance-wise. I believe that it was simply discarded near the end of fabrication, for failing some gauging test, was scrapped, and acquired later by a surplus dealer.

Again, just my opinion - YMMV.