You can find details of the M42 mount Pentax SLRs here

Apparently there are three versions of the S2. Two sold worldwide (the later model has a notch on the shutter speed dial to engage with the clip-on light meter) and the S2 Super which was Japan-only and sold instead of the S1

I have an SV and a couple of Spotmatics, but mostly concentrate on the first generation K bodies and lenses as they can be used without an adapter on those new cameras some people here get very sniffy about. I actually find the SV and clip-on meter easier than the Spotties as you don't have to peer into the viewfinder with the lens stopped down and try to see the indicator needle. With the clip-on meter you just switch it on, set shutter speed, and set the aperture as the meter suggests.