As I recall, both the semi and the full auto version of the Macs have their selector on the left hand side of the grip for right thumb operation. Gordon Ingram didn't cater to we southpaws.
Regarding the firing pin, I was under the impression that the open bolt fired from a fixed pin. Like Jim says, it was easier to convert into full auto and was reengineered to fire from a closed bolt. Hence the higher price for the former in semi-auto as opposed to the latter. Good luck.