re: the 20ga spring bring a replacement from another gun,,that statement I made only from looking at the spring and comparing it to the 12ga in the other lock.
I'm assuming the 12ga spring is an original,,,it is much more graceful in shape and has gentle bends to the legs to better position the ends upon the hammer and the sear surfaces. The bend is tighter, the overall thicknesses (not width) of the spring anywhere along it's length is quite thinner than the 20ga.
Just an observation, might be all wrong. But it jumped out at me as a replacement from another lock, another gun.

The lower grade Meriden guns didn't have the attention to fit and finish a higher grade had. They made some beautiful high grades, the wood quality right up there with LCS and others. Wm Gough was their best engraver.
Yes the bulk of them were made to be sold at a low price in large quantitys. Final fit and finish usually suffer in that quest.
The LCS can be less than well finished inside as well and itself is nothing more than a backaction lock shotgun,,the same thing the Aubrey/Meriden is.

I've owned more than a few Aubreys & Meridens both shotguns and 22rifles. Even still have one of their Meriden 12ga pump shotguns.
One of the highest grade SxS's often shown on the Meriden website(s) was mine at one time. Gough engraved, 32" bbl, pat. pop-up mid bead sight.
Fun to shoot skeet with that one!

I still stop and look a Meriden over whenever I see one at a show or shop. If it's interesting, or one that can be saved,,it comes home with me to do some work on.
Hope you didn't think I was bashing them!,,I rather like their line-up.