if it is an original spencer then it sounds like a hathaway or S&S firearms conversion.
With a spencer the numbers do not indicate bullet or bore diameter or powder charge. They represent the dimensions, in hundreths of an inch of the cartridge case's base and mouth. So a 56-50 has a base of 56/100 and a mouth of 50/100
www.starlinebrass.com sells 56-50 brass, it is intended for the new spencers sold by taylors, I don't know if it will work in a converted original, I do know that you can make the the brass from cut down 50-70 brass. Here is a good link to an article that details the shooting of an original 56-50 spencer, it covers the conversion, making the brass and loading the cartridge
http://www.civilwarguns.com/9204b.htmlload wise, a 350 grain bullet over 45 grains of Black powder is a common load but make sure there is no air spaces in the cartridge, i always use a compression die,or a drop tube. If you need to, use a wad to fill the case so that when you seat the bullet it slightly compress the powder charge.I have seen people split the barrel of a rifle by using a squip load, a light bullet over 10 to 15 grains of BP. They left an air space and the bullet acted as a bore obstruction.It is best to slug the bore before you reload, then size the bullet accordingly .001 or .002 oversize. Make sure you use a flat nose or round nose bullet. The original spencer round was loaded with a pointed bullet which was okay because it was a rimfire round. If you use a pointed bullet with a center fire rnd, the recoil could cause a round to detonate in the magazine.
