Introduced at Photokina in 1954, the Leica M3 established the M-system camera. The 39mm screw mount was replaced by the M bayonet, and the previously separate viewing and focusing systems were combined in one large rangefinder/viewfinder. Its approximately 0.91× finder provided bright-line frames for 50mm, 90mm and 135mm lenses with parallax compensation. Early cameras used a double-stroke film advance before a single-stroke arrangement became standard. More than 225,000 cameras are represented across the recorded variants. The M bayonet and combined rangefinder principle introduced here remain fundamental features of later Leica M cameras.