Short M42 and C to Micro 4/3 adapter. Purchased on eBay.
Pros: Solid one-piece construction. Cheap. Versatile. UBecause of its registration distance much shorter than the standard M42, useful for a number of special applications, e.g. attaching M42 focusing helicoids to Micro 4/3 cameras. Machined groove prevents shorting the camera's electrical contacts.
Cons: Black anodyzed aluminium rear bayonet, integral with adapter body. The C mount is narrow and might vignette in certain applications. A standard M42 lens attached to this adapter cannot focus because it is too close to the camera sensor.
This is a unique combination of M42 and C female mounts. This adapter is an interesting building block for experiments and special setups. No other M42 adapter that I know of has a lens mount so close to the camera sensor.