This is a nice looking bit of kit, but its not the correct thickness. When measured flange to flange its 25.4mm but for old M42 full frame lenses to work on Micro Four Thirds it needs to be around 26.2. if you take the old full frame flange to sensor(film) distance of 45.46mm and subtract the MFT flange to sensor distance of 19.25mm you get the 26.2mm required. To get this adapter to work I had to unscrew the lens approximately 1.0mm to get infinity focus to the correct place which tally‘s with the maths.
This means when you set your manual lens to infinity focus you will get a blurry image. You will need to set your focus to 1.5 meters with a 35mm lens or 15 meters using a 135 mm lens to compensate or make some kind of packing washer to move the lens further away from your sensor.

Using the argument that it is designed to allow for lens tolerance is wrong, after all the old camera bodies worked fine. Plus camera bodies and lenses throughout the years were never manufactured to such sloppy tolerances. If this adapter was 26.1mm everything would be fine, the infinite focus would be so close to where it should be, as would all the other focus distances marked on the lens that there wouldn't be an issue.