Not to muddy the waters, but the PC seems to me to be directly related to the Saxon white tie tailcoat, which is usually worn with a waistcoat. I believe the cummerbund derives from sashes. There may be some connection or there may not.

I notice the Australian solider above is also wearing a sash- not a waist-style sash, but an across the body sash. Does anyone have any experience with such sashes, other than for merit badges?

Is an across the body sash usually made in a loop ( as with the BSA model) or is it a wide ribbon with a closure at the bottom. PLEASE NOTE, I am not talking of tartan sashes, but of the King Edward variety, usually worn as an insignia for some order...