I have three Marlin .54 stainless muzzleloaders.I swear T/C made them for them but both say no.The reason is that T/C parts will interchange with it.I like the under the thumb safety on it.I dont like that it uses caps.They are hard to find in Alaska.I converted them to primer adapters but its a pain to load them.Its almost inpossible with a scope on it.Its a light gun.I think about 100 grains of powder is max with this gun.It might go 120 but I dont trust the primer adapters above 100.I am trying to figure out which T/c it was that bolts interchanged.I think it was thye firehawk.It would be the perfict alaska gun with the right primer adapter.Its stainless .54 and has a plastic stock.Its light as heck just over 6 pounds ,fun to carry not too bad to shoot.I want to get some more .54 caliber sabots that shoot .500 bullets.Thats perfict for moose.I take the scope off to hunt with if you cant have them on.

I had one of those in the .50 cal many years back. Like you, it's a bugger-bear to place a cap on that nipple when you've got it scoped. I shot it for 5 years before winning a knight in a raffle or something. I've since tossed the Marlin.
As for who made it, it was made by Marlin. They bought or licensed some parts from other makers hence the interchangability of the gun.
From what a friend of mine told me, it was never a big seller for Marlin, hence the reason it was dumped.
I remember seeing Bass Pro selling them for $100 at one of their fall classics and the .50 cal disappeared in a few minutes, the .54's took the rest of the day (not many moose in Missouri).