Hopefully, there will not be much of a gap. Put a small straight edge on both the pan, and the frizzen bottom, after you remove it from the lock, and turn it upright, of course. Figure out which surface is the villain. That is the one you file, or grind.
Sometimes it will be the location of the pivot screw for the frizzen. If that is drilled in the wrong locations- too low will raise the front edge of the frizzen, too high will raise the back edge of the frizzen- you have some options. Order a new frizzen is the easiest. The next is to have some machine shop weld up the hole and then drill it in the proper location.
Sometimes the hole through the bridle or the lockplate on locks that don't have bridles, and then you have a different set of problems. If its the bridle, grind the bottom of the frizzen to fit. Its way to expensive to be fooling around re drilling the bridle, and you usually will have to replace the lock plate to get a new bridle. If is a lock without a bridle, you do have the option of welding up the hole in the lock plate, and redrilling it in the proper location. And, I suppose, if you have access to a proper machine shop, you can weld up both holes in the bridle, and redrill them both. That gets to be really delicate and hard work, and it can get expensive- enough so that buying a new lock makes more economic sense. The only justification for doing all the redrilling work is if you are trying to salvage an original lock that has some notoriety or fame, either because of the maker, or the gun it is on, etc. If I had an antique gun worth saving, I would redrill those holes before grinding or filing either the pan or the frizzen. The relocation will be only slightly off center of the original hole, which means that screw head will often cover up the repair work.
