I have tried to get a handle on exactly what was used on early M14 scoped rifles, especially in VN and shortly after. There is much conflicting information out there.
There is an unaltered XM21 in Australia but all I can recall about it is that it had a oblong knob, vs round, to secure the mount. Some sources insist that the Art 1 was not fully ready in time to be used in Nam but others say it was used. I have a numbered mount which was supposedily military and it accepts a M1C mount and scope on an M14. Was this tried as the XM21 was being developed?
Personally, I am in the fog about what was used on the M14s prior to our exit of VN.