I think the marking is Sportbuchse, with an umlaut on the "u". It means "Sport Rifle" in German, so that would seem to point to the origin of the little gun. The caliber could be .22 (short or BB cap, most likely) but also could be 6mm. Some of this type of guns were used for indoor target shooting; the cartridge contained no powder, only the explosion of the primer drove the tiny bullet out of the barrel.
If you look at the barrel, maybe under the forestock, you will probably find German proof marks. There will probably be a crown over a letter and some numbers.
Some of those little rifles have no locking mechanism at all, depending on the momentum of the falling hammer to keep the pressure in. Even if it has what seems to be a decent lock, it is unlikely that the rifle was made after the advent of modern steel, so I strongly recommend it not be fired with modern ammunition.
Jim