How did a rare US Senate book bearing King Charles’ name land in a used bookshop?

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Even at first glance, it was an unusual thing to find in the stock of a British second-hand book dealer: a leather-bound copy of the rules and procedures of the United States Senate, one of 450 printed for the nation's 91st Congress in 1969.

But what made it truly intriguing were the gilt letters stamped on its front: “His Royal Highness the Prince of Wales.”