It is an inspector's mark, possibly for the factory at Suhl. It did not have to be made there, but was tested/checked and accepted there. Zella are nominally Austrian firm, but there are a number of hyphenated firms with that name like Zella-Mehlis, who later linked to Walther and Anschutz around the city of Suhl. I also noted the top of a 'Duchy' type German acceptance mark higher up. (Germany at the time was a collection of dukedoms and small kingdoms and only became 'Germany' after 1871