This won't fit because it's for AR15/10. But it looks very much like the one on the M110A1.
Yet another part I skimped out on my clone for. But I didn't mind. Not sure if someone could find a batch in Europe and send them over here. Im sure that wouldn't be very cheap...
Could this possibly be one of those parts HK USA adds to the rifle once it arrives in the US? Several articles mention that the Army specified many components to be added once they arrive to GA.