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The Mirex mounts on the EOS body it the same manner as any EF lens, and removes the same, as well. There is a very small red index dot just on the top outside edge of the fixed EF mount part, at the "bottom" of the adapter. You have to look at the bottom part of the adapter from the side to see it (it lines up with 12 o'colck on the EOS body). The tilt adjustment is the small thumbscrew that's just below the wide flange of the M645 (or HC) mount plate, at the "top" of the adapter. You tighten this screw to lock the tilt movement, and you loosen it to enable the lens to tilt. I normally use shift to slide the M645 mount flange away from the tilt screw before using the screw, as there's very little clearance between the screw and the M645 mount flange above it. There are two thin, serrated tooth grippy-sliders on opposite sides of the adapter, just above the EF mount part - you slide one of them to the right (if you're looking down at the slider) to release the rotation lock on the EF mount, so you can rotate the Mirex around the camera mount - it has 15 deg click stops. The M645 lens release is the little metal tab on the very top level of the M645 flange, opposite from the small M645 lens mount index dot.
I think the key goes in the little allen key hole just next to the tilt lock screw, and it's probably to adjust the tilt resistance. It's not so easy to tilt the adapter when it's just in your hand, but it works well when mounted on a camera, with a lens on it.
It's a great tool. Give me a shout if you have any questions.