The replica Santa Fe Hawken rifles had this feature. They had a allen socket head set screw in them.
Hungry Horse
High quality long range muzzle loaders had the breech drilled in on side and out the other and many
were low in relation to the bore so the nipple angle could be established.The nipple seat was drilled
usually to the depth of the threads needed for the nipple.The much smaller flash hole would intercept
the cross drilled hole which was threaded and the right side was plugged with a threaded rod and then
upset like a rivet and filed smooth and frequently was engraved.The bolster side had another thread
screwed tightly in and many times covered with a platinum plug sometimes called a "blowout"plug.
Some of these were vented.The idea behind all this work was to prevent high pressures generated by
launching a 500+ grain bullet from damaging the base of the nipple or prevent it from blowing out altogether.
I think the breeches being discussed here were made cross drilled as a production expedient and not an attempt to emulate anything of quality.I know this from seeing X rays taken from an original Alex Henry
long range rifle that belonged to the late Don Brown in Canada in 1987.
Bob Roller