How old is it? It has a domed primer.
I can only guess that this 6.35 mm cartridge was made before 1927, when a working agreement between Geco and RWS left Geco making the pistol ammunition for both companies. Also, it does not have the Sinoxid Primer “O” marking on the cup. Again, this is just a guess. This headstamp of “N” appears as late as 1933, but that was on 9 x 19 mm and has the “O” primer. It also appears on 7.65 mm Browning and perhaps others that slip my mind at the moment.