There have actually been Colt SP1 and PAWS and some other conversions right around the 10K mark on the NFA ad boards at subguns in recent months.

auto sear pin holes and relief cuts are the defining feature of an M16 machinegun receiver.

The fire control group, bolt carrier, and (on some receivers) relief cuts on the upper are required for an M16 to operate properly, but not specific to the M16 alone, and by themselves would not make an AR-15 into a machinegun.