I have built about 12 sidefolders and yes some are really tight and some need "fitting" The tight ones are usally because the hole in the receiver and the hole on the trunnion are not lined up just right and it causes the cilender part of the locking meckinsm to bind ever so slightly. Put a long stone on your dremmel and polish/enlarge the receiver hole and trunnion hole working it up and down and around and around. The locking lug of the catch is likely too long as you say. I have seen it. Looks like poor quility control on the parts or we might be getting factory rejects. Just keep working the hook/wedge part of it down until it works, that;s what I do then I put some Navel Jelly on the freashly cut metal to lightly phosphate coat them. I just keep playing (fileing and polishing) until it works the way I want it to. I hav'nt ruined one yet.