P635 headstamp mark Otto Eberhardt Patronenfabrik (English: "Otto Eberhardt Cartridge Factory") was a munitions company established in 1860.
The company's Hirtenberger Patronen, Zündhütchen und Metallwarenfabrik (English: "cartridge, primer and metalware fabrication in Hirtenberg") near Wiener Neustadt (proofmark "am")[1] used forced labor during World War II from a sub-camp of the Mauthausen-Gusen concentration camp and produced ammunition including 9×19mm Parabellum (pistol and submachine gun) and 8 mm Mauser (rifle) cartridges.
The company also had a factory in Ronsdorf near Wuppertal (proofmark "ap")[1] which produced rifles.
[2] Otto Eberhardt Patronenfabrik also purchased the assets of the Hopfner aircraft company in 1935.
They continued production of both de Havilland- and Siemens-powered aircraft under the Hirtenberg brand.