I'm way late to this thread but thought I'd mention that all the /416\-marked, BTC-imported "M"s that I've seen, including mine, have the /416\ arsenal marking etched onto the receiver rather than "stamped" like the rest of the receiver markings and serial number. I don't know when the /416\ was applied but I doubt it was at the same time as the rest of the stamped markings.
firstchoice