Hey up Ged
The threaded cup type thingy that's held in place by a hollow bolt type thing are missing..
If the trigger doesn't line up with the cylinder then the hollow bolt is loosened and the threaded cup is rotated until they line up..
There should also be a pressed grip washer that goes between to stop them coming loose as it's a devil of a job seperating them if it does come loose!!
Good luck with it, there are a lot of folk that are really in to the Merc's and Airsporters
Here's my Mercury and my Mercury Challenger (In the middle)..
Cheers, John