OK. Not all troops carrying the M-14 were issued the happy switch, though all of the rifles were full-auto capable, according to my readings.
I went through boot camp at MCRD San Diego in ‘69. We were issued M14 rifles, but the full auto selector was blocked. I have had the opportunity to fire a full auto M14 at the Albany machine gun shoot in Oregon before it was shut down, and it’s easy to see why the military gave up on the idea of using the M14 as a squad auto. I have also fired a full auto FAL at the same shoot, and while it was also difficult to control it was superior to the M14.