Many thanks Beth!

Bryce, the badge is definitely not a regimental badge: we know from footage of the performance that full orders were worn that night.

Link to the Pathé video:




(By the way, the close-ups of the Brazilian diadem are excellent, and I think I am growing fonder of the original version each time I see it.)

I think HM was trying to adapt to the lessening of the level of formality of these occasions by wearing "diminutive" Garter insignia.

The other known time she wore the badge, at another film performance in 1969, was with her family orders.

In 1970, Orders were worn as well (no sash but apparently a full star):



From 1971 onwards orders disappeared from Royal Film of Variety performances.