I have contacted the owners of the Republic Metallic Cartridge Co. and they are willing to add 56-50 Spencer to their black powder ammunition line if there is enough interest.
They already load 10 gauge, 12 gauge, 45-70, 45-90, 45 Colt, 45 Schofield, 44-40, and they are tooling up for 44 Colt and 44 Russian.
Please drop them an email if you would like to see this cartridge loaded, and while you are there check out their other ammunition. If you have an old Damascus barreled shotgun, or a 45-70 black powder rifle, they have your ammunition.
http://www.republicmetallic.com/Just my .02
Buck