This helicoid focuser comes as two parts: M42 to M42 17-31mm helicoid, and the short M42 (c-mount) to Micro 4/3rds adapter, which has 25.5mm and M42 female threads. This allows for use of 25.5mm filters, which can come in handy.
Cost: about £30 from Hong Kong
Weight of the M42 (c-mount) to Micro 4/3rds adapter is 22.9g
Thickness of the M42 (c-mount) to Micro 4/3rds adapter is 5.65mm. It extends by 14.09mm in total.
This adapter is very useful and fairly light, especially the fact that it can also be used with a 25.5mm filter (which fits in the c-mount female threads). However, the adapter with helicoid is too thick to allow for infinity focus with my 35mm F3.5 enlarger lenses. There is only a tiny amount of play with this adapter. I like this helicoid focuser and use it very often. Definitely worth getting.