The screw mount commonly called ‘Pentax screw mount’, which was really first found in Praktica cameras, is more correctly called Universal screw mount or generically, M42 mount (according to the threads). Lenses in this mount are plentiful, cheap, and often excellent, and can indeed be mounted on most SLR cameras with one adapter or another. Drawbacks: with an adapter you’re almost guaranteed to need to use stop-down metering (Adaptall-2 is a rare exception).
Cameras that natively used M42 mount (and whose lenses are widely available) include Pentax, Praktica, Zenit, Yashica, Petri, Cosina, Vivitar, and Ricoh.
Konica AR notes: one of my favorite adapters for its simplicity and ease of use. It’s a solid ring with a thick knurled outer part that helps you easily twist it into place on your Autoreflex. Then screw your M42 lens into it. Stop-down metering only. Great way to get a classic WA or zoom on your Konica. (see image above)
Pentax K notes: probably my least favorite and least-used adapter. A thin ring that attaches directly into the K bayonet mount, putting the M42 lens at the same level as the bayonet mount lens. Requires a little included spanner to attach and remove. Fiddly. K mount lenses are also plentiful and inexpensive, and I have enough good ones that I’m never tempted to attach M42 lenses to my K mount cameras. NOTE – I heard from a reader that using one of these with a *Ist camera can be problematic unless you first “break” the spring-loaded portion of the adapter so it doesn’t get stuck with your lens on the camera. YMMV.
Petri bayonet notes: Simple and easy to use, just fits in the bayonet and accepts screw mount lenses. BUT – the Petri bayonet mount utilizes a collar that sticks out where the M42 lens wants to seat. So the only way I’ve found to actually use the thing is to put a small-circumference 2x adapter on, and mount an M42 lens to that. Otherwise you’re better off getting a later Petri with M42 mount.