Originally posted by jaccy60
That's the one, it has a silver end mount with a black and red dot on it
That sounds like it already has a K-mount adapter on it. All the Pentax M42 screw mount lenses I've ever seen were pure black and had no red dot.
Does it appear to be a large diameter (42mm) screw thread, or a "bayonet" mount that turns about 1/8 turn and locks on? That lens is definitely from the screw mount era. If it appears to be a bayonet mount, then it almost surely has an adapter on it, or someone has done major surgery to it. If it is an adapter, it should unscrew, although it may take some effort if its been on there for years.
If you could post a picture of the lens mount end, we could tell for sure.