MP-5 is available as "registered receiver", those that accept std pin on MG lowers, "registered sear", those that have a "sear" installed to the semi-auto trigger pack, "registered trigger pack", somebody put a serial number on a MG trigger pack.
There is a few draw backs to the various types, so if somebody knows the address to the HK Pro?? or whatever it is, there is good info to be had there, I have only followed a link off subguns to the trigger pack/sear story.