According to the Library and Museum of Freemasonry, this belt buckle was given to nurses who trained at the Royal Masonic Hospital, a private hospital for Freemasons and other fee-paying patients. The Royal Masonic Hospital stopped existing in 1992 but its belt buckles are apparently still worn by “elite” nurses such as the one who treated the queen.
Remember that King Edward VII Hospital is where Kate got hospitalized due to a pregnancy-related illness, which led to the suicide of one of the nurses.
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