I was offered an AR-15 selective fire by a dealer when they first came out, but turned it down (OK, OK, I know, but the price was stiff for those days, plus the $200).
I also have seen several selective fire rifles billed as M16's but which turned out to be either selective fire AR-15's or built up guns marked "M16" by the builder. That is why I asked the question.
I do know that during the '68 amnesty, ATTD would not accept registration of rifles marked "M16" and would report owners to the FBI for possession of stolen government property. They claimed the amnesty was only on the machinegun possession, not on theft, though I don't know if anyone went to jail.