The 0 is the seat size which would be 16" on a Kieffer saddle. Not sure about the 5 though Kieffers are set as standard at 32cm width of tree when made but I don't know how that translates to N, M, XW etc. There should be a serial number embossed into the leather near the girth straps - if you email Kieffer they should be able to look it up and tell you the fitting, name, year of manufacture etc.