"There have been a couple of other examples that have gone to auction or sold in the past, but we are talking about a handful," Kimberley Day, the head of coins at RWB Auctions, told Business Insider. "It's very, very rare," she added.
According to the auction catalog, the coin is one of a handful of trial coins struck in 1981 'as part of an experiment' while the Royal Mint tried to find a good denomination between the existing 10p and 50p coins.
Day added that the seller remembers getting it in his lunch change about five years ago, and thought it looked unusual. He put it aside 'knowing it was something interesting,' and pulled it out to be examined a few years later when a similar coin made the news.
The coin is so unusual that there is barely any reference to it online, Day added. But it's had huge interest from coin collectors.
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