Pentax K mount 28mm f2.8 lens. Its Pentax's Comiscar brand (usually reserved for their c mount lenses) but appears to be identical to the Pentax/Takumar version. It's a KA mount, i.e. is an A series rather than M which makes it more useable on modern DSLRs. All works well, no fungus, smooth focussing etc.
eBay