Its a commen caliber for the Lee Sporters. It was a caliber they also called 500/320...but it had never anything to do with any ".500" cased cartridge. Mantons was one, that offered that caliber to their sportsmen. I have encounted quit a few now here on the "net", but all of them, like this one, was a plain vanilla. I have seen them in real delux with express-sights, barrelrips etc, but only in old catalogs.
The ballistic of the 500/320 cartridge was a 244 grain bullet @ 2000Ft/sec.