Queen Elizabeth II on horseback. Likely from the 1951 photo.
Trooping the Colour is a ceremony performed by regiments of the British and Commonwealth armies. It has been a tradition of British infantry regiments since the 17th century, although its roots go back much earlier. (Wikipedia)
Over 1400 parading soldiers, 200 horses and 400 musicians come together each June in a great display of military precision, horsemanship and fanfare to mark The Queen’s official birthday. What happens during Trooping the Colour? Read more on the Royal Family website.
The above is based on the National Button Society's Classification System.
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