The adapter for M-42 to the EOS mount are very solid and they lock on. Have quite a collection of the M-42 lenses and they are built to last. The problem I have using them on my FD mount is the brass centering screw that locks the adapter in position but it usually falls off after a certain amount of handling. You can pick up EOS bodies for cheap. I also notice the P adapter allows the M-42 lens to mount a bit closer to the mirror so that is probably will allow infinity focus. I really had no use for infinity focus for the years that I used the M-42 and with the great FD lenses for sale it never bothered me.
Ya like he said above while I was writing this...:o)