Thanks very much to Steve Smith who posted me some frosted mylar.
As far as I can tell, using a loupe on the mylar, the camera is fine. It looks like the (Vivitar) m42-K/mount adapter was off.
When I used a different -- Pentax brand -- adapter with one of my Pentax lenses it seemed to be perfectly in focus on the film plane and through the viewfinder.
Looking through a 135mm lens wide open there seemed to be good agreement between what I was seeing on the film plane and through the finder.
I don't quite understand why this should be the case - if the distance from the film plane to the lens is off because of the adapter shouldn't the distance to the focus screen be off by the same amount?
Looks like problem solved though.
Anyway, thanks again to Steve Smith and everyone for the advice.