TC offered a scope mount that used the two rear sight holes to mount the front of the mount and the third on the breech to anchor the rear of the mount. With this mount no additional holes needed to be drilled and tapped.
I can't believe that screw is for mounting a tang peep. It would prevent the barrel from being easily removed for cleaning. Now, those sights that SMO posted pictures of do look intriguing. Some years ago, I put a shotgun bead in one of mine. You just lined it up with the front bead and you sorta had a set of sights.
I don't recall ever seeing or having a T/C sidelock with this hole, however, I just pulled out my T/C catalog #21 copyright 1994 and read about their Quick Release (scope) Mounting System. This consisted of a base (and base adapter for different models) that was mounted to the gun and then a tilt and spring ring assembly that snapped onto the base. This allowed you to move your favorite scope from gun to gun with only extra bases. Additionally it allowed you to remove the scope from your muzzleloader during your cleaning process.
Although the catalog doesn't mention the hole you have pictured, the pictures they illustrate with would indicate that Mulemauler is right on in stating that the hole in question and the factory rear sight mounting holes is how the system mounted.
Mulelauler is correct. I remember stocking and selling a bunch of these in the early 90's.
They where expensive in their day and was only sold for 3 or 4 years
Here's a picture from T/C catalog #19 from 1992.