Could be the last, the order of dress for this Royal Banquet stipulated 'white tie', which is traditional for royal functions.
For RAF officers, the equivalent is Full Ceremonial Evening Dress (No. 5A SD), which includes a white waistcoat and white bow tie; for officers of the Royal Navy, I believe that it is No 2A Dress, which includes a white waistcoat but a black bow tie. There will be a plethora of equivalents for the Army, of course!
No 5A used to be worn at Officers Mess Summer Balls; however, the practice was discontinued many years ago and is now, I understand, only required at evening functions if royalty is present.