London, England (CNN) -- People in Britain are being urged to check their wallets for some rare loose change that could earn them some serious cash.
A batch of 20-pence coins were issued in 2008 without a date, and those coins have now gone up in value for collectors, according to the London Mint Office, a coin marketing company.
The company is offering to buy these undated coins for 100 pounds ($154.43) each -- 500 times the coin's value.
Anyone hoping to take advantage of the offer must register with the London Mint Office, which is only accepting 10,000 registrations.
The date error happened after the coins were redesigned in 2008. The distinctive heptagonal coins used to have the date on the reverse, and the new design put the date on the front, on the border surrounding the queen's head.
But nearly 250,000 coins were printed with the new back and the old front, resulting in no date at all appearing on the coin.
The Royal Mint, which is responsible for issuing British coins, says the erroneous coins remain legal tender with a face value of what is written on the coin -- 20 pence.