Hugh- Your guy work on Moss 500's, or just the expensive stuff?
I have a problem with the trigger group, and then disassembled it accidentally. The original problem I had was after firing/pulling trigger, I couldn't chamber a round/cycle the slide unless I activated the release button on the bottom. I disassembled & cleaned the whole action, then spray-cleaned the trigger group. I got the trigger & release button to work properly out of the gun, but when re-installed, it would lock up 50% of the time.. I pulled it again, & popped the wrong pin out & the springs all let out..

That's why there are gunsmiths.... yeah he works on them... bring him all the parts you have and see what you lost.... he is there Weds thru Sunday most days till 1:00 ish.

or bring the gun is and I will tag it for him with a baggy of parts if he is not there. or better yet bring it and the parts out and if the smith is not in I will have Mike the instructor see what he thinks about your trigger and maybe he can reassemble it.