The EOS 3 that I use for M-42 lenses takes a bit of sanding/grinding in the one area of the switch on the adapter but once you get everything set up it is a pleasure to use. I have a few M-42s in my small collection which I would rather use than some of the EF lenses. Here is my favorite adapter:

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There is a small tab that prevents the adapter from coming off unless you press the button on the camera. I experimented with a few of the cheap ones that did not work as well. It also has the lens stop so you can not over tighten it to do mirror damage. The one post with the adapter looks the same. Of course I don't have the chip but I use it manually with no problems. By the way the grinding/sanding is ONLY on the adapter NOT the camera in case I worded my explanation wrong. The M-42 is always a very good build with metal instead of plastic EF