After further research, and getting some help on another forum, it's looking more like a black Asahi Pentax Spotmatic. It has two PC connections on front for bulb and electronic flash, and the Meter On switch can just be made out on the upper side of the lens mount.
Amended: It's an SV, not a Spotmatic. I've been told that at the time the photo was taken, the introduction of the Spotmatic was still months away. That, and there is no battery cover on the bottom (the SV had no internal meter). So I take it that is not a meter switch I see in enlargements of the photo, but may be the bottom edge of the aux meter that clamped on top of the prism housing.
Waiting for the light