The photo was taken during a shoot lasting just a few minutes, and it is said to have been one of the last frames after the king finished posing and went straight back to work.
He sits at a 19th-century French mahogany desk, in front of the painting Jacob and Leah with their sons, Francesco Zuccarelli 1743, purchased for the Royal Collection by George III.
The red box stands on a gilded wooden stool, made by Henry Williams in the mid-18th century.
In the background is a striking black and white photograph of the late Queen and Duke of Edinburgh, which the young couple gave to King George VI for Christmas in 1951.
New red boxes from him are currently being made for the king, with six boxes to carry his royal code.
They will contain documents from government ministers in the UK and the wealthy, as well as Commonwealth representatives, sent from his private secretary’s office to the king wherever he is in the world in a sealed, red mailing box. .
Beside him are more papers, one of which is tied with a ribbon.