No-brand M42 to Micro 4/3 adapter, purchased on eBay.
Pros: Cheap, possibility to release the set screws and adjust the position of the M42 mount. Flange at the back of the M42 mount keeps the lens aperture pin pressed, thus allowing the use of M42 lenses with automatic aperture even if they don't have an auto-manual switch.
Cons: Micro 4/3 bayonet is black anodyzed aluminium and integral with adapter body. Interior is relatively shiny and may need to be painted flat black or lined with flocking to avoid flare and loss of contrast. No possibility of using the auto-manual aperture switch on the lens to achieve a quick preset aperture (the aperture pin of the lens is always pressed). Two screw holes at the bottom of the adapter are presumably intended for mounting a tripod attachment, but no such accessory was provided. The holes are closed with a friction-fit rubber cover that may fall off and be lost.