I'm getting older and my memory is far from perfect, but I believe many years ago T/C offered the Duo Ring base for forward mounting a long eye relief scope on sidelock muzzleloaders. It sure looks to me like that is what is pictured. The holes should match the hole pattern in the T/C adjustable rear sights (unfortunately I can't find a spare sight in my parts collection to measure). You removed the adjustable rear sight and installed the base in it's place.
Yes, T/C also made the Duo Rings for mounting on other models (Contender. etc.) and different configuration (quick release like the eBay link), but I'm fairly sure the one you posted is the old rear sight replacement model for an octagonal barrel.
Actually, the forward mount just might be advantageous to me these days. It would keep the scope away from the blowback at the breech, and offer the ability to mount a scope so I could see the sight and target at the same time!