For a comparatively short time span in the 1950's the Royal Canadian Navy used metal collar badges for the ranks of Petty Officer and above.  Here is a collar for a Chief Petty Officer 1st Class.  It measures 20mm across and has two clutch pins on the back.  These aren't around in great numbers any more.

This pair is in excellent used condition.  The manual page shown is from BRCN 213.

Good luck and thanks for looking at one of The Mighty Atom's sales.