the asahi optical company took a different manufacturing path, inspired by the german slrs. its first model, the asahiflex i, existed in prototype form in 1951 and production in 1952, making it the first japanese-built 35mm slr. the asahiflex iib of 1954 was the first japanese slr with an instant-return mirror. previously, the mirror would remain up and the viewfinder black until the user released the shutter button. in 1957, the asahi pentax became the first japanese fixed-pentaprism slr; its success led asahi to eventually rename itself pentax. this was the first slr to use the right-hand single-stroke film advance lever of the leica m3 of 1954 and nikon s2 of 1955. asahi (starting with the asahi pentax) and many other camera makers used the m42 lens mount from the contax s, which came to be called the pentax screw mount. pentax is now part of the ricoh corporation.