M42 (AKA Pentax M42, Pentax Universal) refers to a 42mm x 1mm metric screw threaded mount that was widely used before being superceded by bayonets on most cameras from the 1970's on. An M42 mount is very handy because just about all interchangeable lens cameras offer some sort of cheap M42 adapter. So you can set up as follows:
Adaptall Lens > M42 mount > M42 adapter > Camera.
Pic 1 shows the standard and easily findable M42 auto aperture mount with pin.
NOTE that if using this mount with an M42 to PK adapter, unless the adapter depresses the pin (or the mount is 'modded' so that the pin is depressed) the adaptall-2 and SP lenses
won't stop down. Only the earlier adaptall - 1 lenses have an Auto-Manual A-M switch with which to stop down manually.
This thread discusses ways around this. Another option is to keep the iris actuator (lever with a small wheel) on the lens side depressed see
here.
There are also 4 other M42 variant mounts with aperture connections for a specific camera: the Pentax ES (pic 2), and ones specifically for Mamiya (pic 3), Fujica and Practica. When not used on these specific cameras they all work the same.
The mamiya M42 mount in pic 3 was jammed on a lens. The problem was a slightly bent auto aperture pin. This was stuck all the way in, causing the aperture connection underneath to catch on the light baffle on the lens. Forcing the mount off did not cause any damage of significance to the lens - only a bent light baffle.