My .02:

The 12.5x70 MB is an old rimmed target cartridge (BP) that has absolutely nothing to do with the 12.7 x70 Schuler or the 500 Jeffery.

The 12.5x70 MB "Mauser boden" was made between 1880 - 1910.

There are modern 500 Jeffery cases marked as 500 "Jefferies"
There was also a gunmaker in the UK by name of Jefferies as opposed to WJ Jeffery and Co. after William Jackman Jeffery.