In regards to notching the breech face, some historical reference is needed. No, I can't quote page and verse of the reference, however this is the gist of it.
Flintlock military firearms had a drilled touch hole even with the breech face. As late as the Civil War, when firearms were brought in from the field, many were "percussioned" using the drum method. Since the touch hole was even with the breech face, it was necessary to notch the face of the breechplug, since the drum would encroach on the breech threads. This operation was called "Onbriching." Literally thousands of firearms were converted using this method.
Thus, it stands to reason that guns which have notched breech faces either had them done as a result of adding a touch hole liner to a flintlock with a drilled touch hole; or, were converted to percussion using the above method.