Definitely a ruler
I've never seen one, but this was my thought. How weird.It's the USB cable strain relief attachment for a 1srs body.
Bob

Nope....it's threaded so that you can put the previously removed dust cap back on.Thanks to @Canon Bob Incidentally the knob bit is surprisingly high quality. It's solid metal, quite heavy, and it is internally threaded so that you can attach... something else to it whilst in use. Not quite sure what though? Another cable strain relief widget?