The accompanying caption read: "Taken 70 years ago this month, this photo shows The Queen, The Duke of Edinburgh, Prince Charles and Princess Anne enjoying a warm summer day in the Clarence House garden."
The post then directed users to take a virtual look around Clarence House, the official residence of The Prince of Wales and The Duchess of Cornwall.
Needless to say, fans in the comments were delighted by the throwback.
"What a beautiful picture," one touched user wrote.
"So heartwarming photo 😍i love your family to beats ❤️," another added.
"The Queen is gorgeous 😍," observed a third.
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In a sweet touch, the post comes less than a week before Princess Anne's 71st birthday, on August 15th, meaning she was almost one-year-old in the garden picture.
Clearly Charles wanted to get in early with the first birthday tribute, but we're sure Anne will receive many more on her big day.