An old (1880s?) bottleneck blackpowder cartridge, one of several designated 500/450. Probably a Westley-Richards offering, if not, then derived from an earlier Westley-Richards. My notes and books are a little murky here. Ballistics comparable to the .45-70 blackpowder cartridge. A couple of my books give dimensions for the cartridge but since they differ slightly, those dimensions are suspect.
CH4D will make a die set (around a hundred and fifty bucks), but the purchaser must provide them with a chamber casting. I got no load data, but I reckon a case full of FFG and maybe a 350 grain cast bullet would be a good starting point.
Most of the info previously corresponded with "Cartridges Of The World" by Frank Barnes. It was loaded in black and smokeless powder versions until 1938 and was used in single shot rifles many based on the Martini action. It was introduced in the early 1870's. It is a fairly effective short range cartridge for North American big game and is similar to the 461 Gibbs Match Cartridge.