Review on Fotasy Canon FD Lens to Canon EOS RF Mount Metal Adapter: Perfect Fit for Canon FD Mount Classic Manual Lens & Canon EOS R Mirrorless Camera EOS R/RP/Ra/R5/R6 by Andrew Mclemore

Works great for M42 lenses that have space behind the mount. But with lenses like the first one pictured above you will run into issues where the aperture ring or focus ring will stick to the front of the adapter so it can be a little tricky as it needs to be tightened to achieve infinity focus . Pictured above with Mamiya 50mm f/2 and Helios 44-2. Comes with a hex wrench that allows you to reposition the threads so the lens distance scale is on top if it ends up in the wrong place after screwing it in. If you have a lot of lenses with very flat backs, you might want to consider a different adapter that has the flange further from the body for better compatibility.