64
Metal, about 1 inch in diameter
(i.imgur.com)
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The hole looks like it's for a set screw to me, but I agree the cap is just a decorative cover. Not sure what this would be used for though as I don't understand what function a collar and cap like this would do mechanically.
Edit: I see OP mentioned the holes don't go through so you're right they'd be for some special tool.