The 6.5 x 57 Mauser was developed @1893 - '94 as a necked down version of the 7 x 57mm cartridge. It was never adopted as a military cartridge by any military power, however it undoubtedly influenced the design of many of the 6.5 military cartridges. (Source C O W 14th Ed page 574.)
What I find interesting about this one I have is the stabs on the primer pocket which I thought was only used on some 303 rounds that I have which apparently was to stop the primers backing out when used in aircraft M.G's (see pic #2)
303 MkVII for scale.