JP - I thought you wrote that the Keller had a 51.7mm case length ?
The Roth version in the earlier post you listed as 50.8mm. Even as I stated Keller/Hirtenberger changed the designation over time.
Both German and Austrian versions of their 10x51-52mm MB cases vary considerably in case length - from 50.8 to 52.11 mm in the examples I am aware of. I think it can be a mistake to think that because two old MB target cartridges vary in case lengths by 1mm that they necessarily are different calibres - the case length does not appear to be absolutely vital in these old chambers. You could double the number of MB cartridges (there are already 154 that I am aware of) if you used that principle. The company tolerances from company to company seem to have varied considerably. Even within the same company there are significant changes in dimensions, although there must have been many issues with chambering different manufacturers products c1975-1900.