Vintage DRAEGER Mann 12l fire vehicle gauge 60ies

Vintage DRAEGER Mann 12l fire department vehicle gauge from the 60ies in used condition. The gauge is without dents, the lens is ok and in total good aged. The diameter is 5 cm and the depth is 3 cm. The display is stamped with “DRAEGER DM 596, Mann 12l/min, ölfrei, fettfrei“. I do not know for what it is. It came from a fire department in Germany with many other “Feuerwehr parts”.