You probably already know this but for the benefit of others, 50-100-450 is an alternate name for the 50-110 WCF. It denotes the original loading of 100 grains of Black powder with a 450 grain bullet. The cases are identical. With respect to the 348, the parent case was the 50-110 so dimensions match up, the major issue is getting the brass to 2.4" as the 348 case was shortened to 2.26. While you do gain some length from straightening the neck, you rarely get the full 2.4". Fireforming works if you have no other options, but buying brass made for the 50-110 when you can find it is far easier. Also, these are not guns you do a lot of high volume shooting iwth, so 100 brass will last a really long time.I own a Winchester Model 1886 marked 50-100-450 on the barrel. Beautiful old rifle. I found some new, primed .348 brass that I fire-formed to make .50-100 brass, but the fire-forming process was not very satisfactory. I still have a box of the brass that was never fire-formed, and I'm thinking I might try neck annealing before fire-forming it. Or I might buy some Starline brass if I can find it. Don't need much -- it's not the kind of rifle you shoot a lot at a single sitting. I have never used it with black powder -- worked up a load with 3031 starting with .45-70 load info, since when I started developing the load (late sixties) there was no load date available for the .50.
The only game it has taken was one jack rabbit at about 50 yards. Explosive results!
To OP: I would be interested in more info and photos of your rifle when you get a chance. I'm guessing you have a Winchester 1886, because I don't think earlier models were manufactured in .50 caliber. That had to wait for the strength of the 1886. Possibly an 1885 single-shot, but I kind of doubt that. Please let us know what you find out!