Um.....the inserts in my parker hale are what look like brass. So in section I think its a Tee with the leg hollow for the screw to pass through. The top of the tee is washer like in function it might well come up against the trigger bracket. There is one either side. I'll see if one is easily removable to take a pic of to confirm what I think I see.

From what you are saying it almost seems best to take the mk2 rear end back to mk1 spec, not a hard job.

regards