to my own surprise the gunmaker hade a Mauser in 9x57 in the workshop. a simple but very, very slender rifle made from Franz Kettner in Suhl. not to confuse with the well known Eduard Kettner Köln und Suhl. But I have no doubt that Franz and Eduard were related to each other.
the hunter now own it got the rifle with other's from an inheritance and did not know what do because 8x57IS clearly dont fit. never heard of a 9x57 and thats all common.
a forgotten round

very rare to find a 9x57, a 9,3x57 and 9,3x53R same time in a shop here