Cosina — Pentax K / Minolta MD 28mm f/2.8 wide-angle prime from the 1981 Cosinon-W series
By 1981 Cosina was selling complete Pentax K-mount SLR bodies under its own name and refreshed its lenses to match, launching the Cosinon-W wide-angle series. The MC Cosinon-W 28mm f/2.8 replaced the earlier screw-mount Auto-branded 28mm as Cosina's standard wide-angle prime.
The lens mounts natively in Pentax K bayonet fitting and was also produced in Minolta MD mount. Period listings record a 49mm filter thread, five aperture blades, around 140g weight, and a 0.25m minimum focus distance, with an f/2.8-f/22 aperture range and MC multi-coating.
Against the earlier Cosinon Auto 28mm f/2.8, this version focuses noticeably closer in a shorter, lighter bayonet barrel, part of Cosina's early-1980s shift toward compact primes pitched directly against Pentax's own SMC Pentax-M 28mm f/2.8 on price.
Because Cosina made cameras as well as lenses, this 28mm turns up used both loose and still attached to Cosina, Ricoh or Chinon K-mount film bodies. Worth checking before buying is that the aperture ring detents still click firmly and that the 49mm filter thread has not been cross-threaded.