I have a trigger identical to the one in the picture & visually the inertia block on mine appears to be the same size dimensionally as yours. The return spring may have been modified on yours though. John
I have never shot .410 with this trigger. I bought an old MX-8 a few yrs. back that was set up for live bird shooting using heavier loads. It would not reliably cycle my 1 oz. reloads on dbls. so I added some lead tape to each side of the inertia block & it worked fine after that. Shot it that way for couple of yrs. & eventually sent the gun to Doug Braker for some R&R & he worked on the trigger to function w/o the lead tape. John
There actually two sized block one for 12/20 and one for 28/410. The cost is about $85.00 for the block. I got mine from Giacomo and installed it myself in an old Comp 1. It looks like the size I ordered.