Gah. I can't find my M42 to EOS adapter anywhere. As I continue to blow the dust off 15 years worth of photography equipment, I've now unearthed my not insubstantial collection of mid 20th century manual focus M42 lenses. I want to mount my E. Ludwig Meritar 50mm f/2.9 to my IR 5D. It's a quirky lens, and fantastically slow by the standards of 50mms. I've shot film through it, but never mounted it to a full frame DSLR. I actually have a bunch of M42/EOS adapters, but they're more or less permanently attached to my (as of, say, 2009) preferred lenses. The adapters are secured to the lens via a set screw, which is nontrivial to remove. The weather is perfect right now, and if I can't find the adapter I want, I may be forced to dig out a micro screwdriver.
Also I have a bunch of photos to post here. Later.