I've never owned a Pentax Spotmatic but the first "serious" camera I ever used was one. I got the photo bug due to those cameras. I really should buy one since they are available so cheap these days. They had some great lenses.
The "universal" 42mm screw mount fits the Pentax directly. It was also known as the "Pentax/Praktica" mount. It was replaced by the "K-mount" which became something of a "universal" bayonet mount in it's day.
There were other screw mounts. The 39mm "Leica thread" mount was used in rangefinders. The "T-mount" was a screw mount that took adapters to use on various SLRs. There were probably some others I'm not aware of.