A recent change of the 20p design meant moving the year date from the tails side to the side with the Queen’s head.
Thousands of coins were minted using the old version of the Queen’s head, which does not have the year date, and the new version of the tails side, which also does not have a date.
Estimates range between 50,000 and 200,000 of the coins remain in circulation.
The London Mint Office, which is a private company with no connection to the Royal Mint, is offering £50 a time to anyone who has one of the undated coins.
Nick Hart, coin expert at the London Mint Office told the Mail: “Mistakes like this not only fascinate people but also have considerable value-increase significance.”
People who find an undated new 20p need to register their details on the website www.undated20p.com with The London Mint Office before sending in the coin and claiming their reward.