Sadly for us, the video has been taken down, so I cannot test my idea.

My recollection is that the brooch QEII wore to secure her sash to the back of her gown was oblong in shape. Unfortunately, I didn't do any stills of that section.

But could the brooch be the brooch part of Queen Mary's chain link choker/bracelets?

Mauriz did a very interesting post in 2018 showing how the brooch detaches from the frame it rests in when worn in the choker.

See https://mb.boardhost.com/historyroyaljewels/msg/1531719685.html

Did anyone do any stills of that segment?