Need Info on Extraordinary WO1 Sleeve Badge

[rickmcm]

I recently bought this extraordinary framed WO1 sleeve badge on scarlet backing here in the US (I'm British but now live in Virginia) and it has the following handwritten inscription on the back of the frame:

"BRIGADE OF FOOT GUARDS - BRITISH ARMY GUARDS REGT - SGT - MAJOR INSIGNIA WORN ON TUNIC SLEEVE - PURCHASED IN LONDON 1978"

The insignia measures 6" high by 5 1/2" wide and is executed in gold and silver thread adorned with gold sequins.

The whole badge is embroidered in high relief with the central shield of the Royal Coat of Arms being about 1/2" thick..!

I served in the Corps of Royal Engineers in the 1970s but never saw such a magnificent insignia.

Can anyone tell me anything at all about it? Date? Regiment? Reason for it having been so exquisitely and expensively made even..?

I have worn No 1's but never saw such a badge, even on those.

Thanks in advance