I think what you have seen was an old cape gun for the mother cartridge 6,5x70R, so called "Bleistiftpatrone" or pencil cartridge - you can write with a soft point load on paper very fine.

there are not many informations about the 6,5x70R, it was for very slim drillings and büchsflinten. ballistic was the same as we have for the better known 6,5x58R Sauer&Sohn. the 6x70R was developt from Norma for Krieghof as a very low pressure round for barrel inserts. this way the power is under the old 6,5x70R because kriegkof dont like high pressure loads in combination guns. the 6x70R is till now the only case with the base diameter of the 6,5x70R, it is simply necked down.