M42 is the "universal" screw thread mount for SLRs that is usually associated with Praktica and Pentax. It was actually used by many brands, including Fujica, Mamiya/Sekor, and Zenit. Because it is an open standard and has a relatively long flange distance, it has also been a very popular mount for adapting to other mounts (notably excluding Nikon F). There are many commercial adapters for M42 lenses to other mounts, including Leica M.
Ok, so why 3D print this one? Because we can seems like a decent answer -- this is an easy print in 1 piece, including the M42 screw thread. However, there are two even better reasons. The first is that an M42 to M adapter allows the latest Sony E mount bodies to autofocus these manual lenses via the Techart Pro LM-EA7 adapter... which has a 700g weight drive limit, so we don't want to waste weight on a heavy adapter. The second is that later Mamiya/Sekor M42 lenses had the aperture ring extend back around the mount flange, but commercial adapters have overly wide flanges that prevent those lenses from mounting. Actually, there is also a third potential issue in that some M42 adapters are too thick on the M side to fit on the LM-EA7. In any case, this adapter avoids all these problems.
The last two shots were taken using the LM-EA7 autofocus with the manual Mamiya/Sekor 55mm f/1.8 lens shown. Note that you can shift the autofocus range closer by not leaving the manual focus ring set to infinity.