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Pentax K body + M42 lens


Not exactly a coincidence, rather a "conscious coincidence".

The flange focal depth of the PK and M42 is exactly the same (45.46 mm). When the Pentax switched from the M42 to PK locking system, the company possibly decied not to alienate their loyal customer base, so they made mounting M42s to PKs possible.

Adapters fit the PK bayonet lock perfectly, making it function basically the same way as an M42:

Release keys are usually included in adapter ring sets, but most of the time they can be separated by hand or a simple door key.

If you plan to use your M42 often on a PK, you'd better consider buying a better quality adapter, because cheaper seem to be significantly less well made.

Certain M42 lenses (like the 50mm ones with brightness feedback) might overhang the bayonet lock. The overhanging lens housing or aperture control mechanism might cause issues with certain EF cameras, since the mirror might get stuck in them. A stuck mirror doesn't necessarily cause more serious problems, but that might damage the mirror, it could fracture of detach. Certain bodies due to their quality and design are less prone to this problem (like the 5DMII), and some are less robust (like the 6D). A possible solution for this issue might be using live view mode if available, this is possible only with those types that don't actually record with open mirror (like the 5DMII), but some newer EF cameras tend to release the mirror before exposure (like the 6D). The smaller mirror of the cameras with APS-C sensor doesn't interfere with the lens.

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In terms of compatibility, we distinguish four separate categories:

This means that you'll be able to apply the lens on the camera in question with a proper adapter, and full focus range will be available with it (often with aperture control too). If AF is supported, you'll see it in the description.
This means that due to the flange focal depth or other factor this conversion would be normally unfeasible, but with some compromises (e.g. using a speedbooster adapter) can be made possible.
This means that the flange focal depth should make mounting and using inifinty focus possible, but for some reason there are no adapter rings available or other factors make it unviable.
This means that you won't be able to mount your lens due to mechanical incompatibility or the difference in flange focal depth doesn't make inifinity focus possible. Although numerous lenses can be mounted on cameras without infinity focus, but this cannot be considered actual compatibility.

Additional information for compatible variations:

manual focus (on the lens, with its own control ring)
autofocus available (AF controlled by the camera body)
manual aperture control (on the lens, with its own control ring)
automatic aperture control (controlled by the camera body)
speedbooster (focal reducer) adapters are available
macro (variable length) adapters are available
tilt-shift (or other flexible) adapters can be used

If you'd like to read more in-depth about compatiblity, you'll need to get familiar with flange focal depth, see this article.