Make sure when moving the spring down into the slot in the post (either by thinning the spring or widening the slot),,that the position of the spring does not change with reference to the engagement point on the hammer.
The upper leg of the main spring and that engagement is what holds the spring in place horizontaly.
The tip of the leg that engages the hammer directly should just barely seat into the shallow notch in the hammer while the back end at the bend sits firmly in that slot in the post.
That keeps the spring from shuffling back and forth during operation but still moves freely in it's arc of motion.
That small mainspring lock screw directly above the spring near the back end keeps the spring in place verticly (along with the lower leg bearing onto the sear).
That screw looks like a replacement (round head screw?), but maybe in the 20 they used a headed screw in place of the headless used in the 12ga guns.
The head on that screw may be helping to secure the mainspring down onto the plate(?), just wondering as that is what it looks like from here. The 20ga mainspring looks like a replacement from some other gun to me but again 20ga Meridens are an unkn.
Have fun,,The Meriden guns are a decent shotgun IMO.
The spring(s) you should be able to file to shape with any decent, sharp hand file.
Straighten out the lockplate first before you do anything else.
