Pentax K mount bayonet was introduced in 1975 to replace the old M42 screw mount
in 1977, in response to the compact Olympus OM system lenses of 1973, Pentax started creating more compact lenses (“Pentax M”) for their new compact 35mm SLRs, the Pentax ME and MX
in 1984, in response to Canon's shutter priority exposure mode, the “Pentax A” lens family also offered shutter priority mode
in 1992, in response to Canon's AF technology, the Pentax FA range of auto-focus lenses were made
timeline
1975:
SMC Pentax K 85mm f/1.8 (1975-77) - optically, same as its M42 mount predecessor
1977:
SMC Pentax M 85mm f/2.0 (1977-84) - not as sharp as the older lenses
1984:
SMC Pentax A 85mm f/1.4 (1984-89) - similar to Zeiss Planar 85mm f/1.4