People living in Birmingham have been the luckiest at finding undated 20p coins which are worth hundreds of times their face value, research indicated today.
The coins became collectors' items in June last year when it emerged the Royal Mint had accidentally minted coins without a date on them for the first time in 300 years.
The coins changed hands for hundreds of times their face value, with one sold on eBay for £7,100.
The London Mint Office, a private coin collector company that is nothing to do with the Royal Mint, offered consumers £50 each for the coins.
The firm said today that it had bought 720 of them since June last year, with the highest number coming from people in Birmingham, followed by those in Liverpool, Manchester and London.
Other areas from which it has received a high concentration of the coins included Hull, Newport, Wolverhampton and Ipswich.
The group has now doubled the amount it is prepared to pay for one of the coins to £100.
It is not known how many of the coins were in circulation, but it is thought to be between 50,000 and 250,000.
The coins are known as mules because their heads side does not match their tails one.
The mix-up occurred due to a re-design of the coins, which involved the date being moved from the tails side to the heads side.
But the old Queen's portrait design, which did not include the date, was used for the heads side, leading to the coins being minted without a date.