Well, guys -- I was beginning to feel a bit beleaguered there, and had decided not to respond/prolong this interchange. But some of you seem genuinely concerned, so:
1. The face of the breechplug determined where the touch hole went. 2. The touch hole determined where the lock went. 3. The bolster (behind the screw) determined where the lock bolt hole went. 4. The lock bolt hole did, indeed, go through the barrel/breechplug threads -- about 1/8th inch into the barrel. Horrors, you say.
My response: I've already mentioned the breechplug (installed by the barrel maker) is so tight I can't (easily, at least) remove it. Drilling a hole through it near the back end of the barrel isn't going to weaken it appreciably. Besides, what could make a VERY snug breechplug any more secure than running a bolt through the back end of it to assure that it ISN'T going to come loose? The bore is .45 caliber. I'm planning to load it with 15, 20 grains of FF black powder, MAX. I realize you helpful folks of the purist persuasion (no sarcasm intended) are horrified that this isn't the way it's "supposed to be done." I don't like it much, myself. But when the situation calls for making do -- in what I consider basically safe fashion -- one makes do.
I came to you asking whether a detente inside the lockplate was a "locator" for a lock bolt. You didn't answer that question, but offered other advice. I -- not blindly, incidentally; I've built more than 30 rifles, all of which function, and safely -- considered the advice, weighed the consequences, and took one suggested solution. I'll shoot the pistol with minimal loads (never anywhere close to heavy), and watch for signs of problems. If there are problems (which I seriously doubt), I'll dismantle the pistol and use the parts for some other project.
I always appreciate the help I get here. I sometimes am slightly irritated by folks being horrified that I don't always do things the "right" way. Won't belabor this further. Thank you all for the advice. As always. -- paul