I suspect the OP is asking about what I would call the "pivot hole", the one that's ~1/16" in diameter, about a half inch "behind" the big hole where the key mechanism comes through. The part that actually blocks the hammer from moving, when you turn the key to lock the gun, is a stud on a plate, and that plate has a small pin that fits into and rotates in the pivot hole. If you want to fill that hole once you remove the lock parts you're going to have to have some way to fix the pin/rod/whatever that you use to plug the hole, so it doesn't fall out (or worse, in, where it will find its way into the mechanicals of the lockwork.) The usual ways to fix a pin in a hole are adhesives or threads, both of which are rejected.
I went through the same mental exercise when I took the lock out of a 638. I used a PLUG to fill the big hole. But the only way I could think of to fill up the pivot hole without permanently affecting the frame was to make a plate that would fill up the cutout in the frame (next to the hammer) that the original lock's operating plate moved in. My replacement plate would have a pivot pin in the proper place, but would NOT have the stud on the back that blocks the hammer. The replacement plate would be held in place by the hammer, and the little pivot pin would fill up the pivot hole.
In the end I decided it would be a LOT of work to make and fit something that precisely, so I have an open pivot hole in my 638 frame.
