Black River Smith,
The Lyman X-17 shellholder is for the 45-70 family of cases. The Lyman X-22 shellholder is the standard 50-70 shellholder and is what is needed for the 56-50. Any RCBS, Lee or other 50-70 shellholder should also work, if these others make 50-70 shellholders. The Lyman is the one that I use.
Hell-Er High Water