My MKII just happens to be in a state of disassembly. Please pardon the putty, tape & sawdust in the photos.
Check the center of the round recoil lug that is dovetailed into the barrel. Tere is a setscrew in the middle of that, although I think it may be a torx screw.
If by hex head you don't mean an allen socket, then it probably comes from the very back of the action on the bottom and is used to help hold the trigger and magazine well assembly onto the receiver. there is some sort of a washer that goes with it as well.
I hope this helps.