A friend of mine has an early .404 made by Mauser of Oberndorf and marked with the same script on the front receiver ring as yours. Which is to say that, it was made before this cartridge was, according to some references, even released. It is, however, the original caliber. The very early ones had slightly different bore/groove sizes. Probably not enough to be an issue with bullet selection, but it might be worth slugging the bore if only for interests sake. I'm not too sure now what my information source was, but Hatari Times had a good write up on the .404 and, if you can find this back-issue, I'm sure you'll find it interesting.