Hi,
This is because the breech plugs used on TRS barrels are deeper than the originals. The lock parts are cast from originals and also suffer some shrinkage during the process. As a result, if the lock is positioned such that the back of the pan fence lines up with the end of the barrel, as on originals, a vent hole centered in the pan will hit the breech plug. This is made worse because if you use a TRS stock, the lock is positioned so the fence is lined up with the end of the barrel, forcing you to notch the breech plug. Lexington1, a notched plug is fine and should be safe. However, let me make some suggestions based on a lot of experience. The barrel wall thickness of a Bess barrel at the breech is wide. You will have plenty of thickness for a liner to be installed without the threads on the liner interfering with the breech plug threads. I suggest filing the notch in the plug bigger and much wider. Make it a big wide funnel shape deep enough at its outside edge to clear the vent hole and reaching right to the middle of the plug face. Smooth and polish it so it cleans easily after you shoot. Next, install a 5/16" diameter White Lightning vent liner in the barrel. Assuming the existing vent hole is where you want it, take out the breech plug, and redrill the hole for the liner all the way into the bore, tap the hole, and then grind the threads on the liner so it goes in only as far as the outside of the breech plug threads. Therefore, it will not interfere with installing or removing the breech plug. If you contact Barbie Chambers, you can get the liners made from carbon steel rather than stainless so they will tarnish and match the patina of the barrel.
You counter sink the head of the liner just a small amount so you can file it flush with the side of the barrel. Finally, for a Brown Bess, drill out the hole in the installed White Lightning liner with a 5/64" drill. Shoot the gun and if you get many flashes in the pan but no ignition, drill that hole to 3/32". I suspect that will make for reliable ignition and for effective cleaning. This recommendation is based on experience and knowledge, not speculation.
dave