Dr Dave, As everyone above stated, it can be any , part , or all of 7 parts, it just depends on what was registered originally on the Fm 1 or Fm 2.
Wildwilly had the disconnect, others may have the trip, or sear, etc.
If you are looking at buying an M2 gun, you simply need to check the data listed on the Fm 3/4 and it will tell you which part(s) are registered.
Hope this helps