My understanding is that FA M14icon's were only issued as per the CO's direct orders and while all M14's had the of being turned into FA, actually adding the "happy switch" could only be done by qualified armorers. Also there are mention in period manuals that all "National Match" M14's were semi-auto only. I suspect that the number of M14's which ever had the switch added was very small overall. If you take a look at period pictures, almost all of them have the round knob instead of the selector.