This is a quick release mechanism that fits the plates found on the Velbon QB-4W. They are pretty cheap and common. You could also print your own.

By itself it is not very useful. I plan to use this mechanism in a mini-projector mount as a means to quickly attach and remove the projector. But, I am sure there are a million other things to do with it. Knock yourself out.

It is parameterized for other square quick release dimensions.

If I recall correctly the screw is an M6x10mm with a 10mm nylon nut on the bottom.