The 35-year-old labourer was given the "lost" 300 year- old Queen Anne Vigo five guinea coin by his late grandfather 30 years ago.
After he bought his son a treasure chest he dug the shiny gold coin out and gave it to him to play with, before having an auctioneer have a look at it.
The lucky owner of the coin does not want to draw attention to his potential windfall and has chosen to remain anonymous, The Sun reported.
Tong said the owner of the Vigo coin, a married father from Hertfordshire, returned the following day to check the expert had not been pulling his leg.
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The coin was one of only 20 ever minted with 7.5lbs of gold seized from Spanish treasure ships by the British in 1702 following the Battle of Vigo.
Of the 20, the whereabouts of 15 are known of to this day, the report said.
In 2012 another of the coins sold for nearly 300,000 pounds at auction.
In a letter of provenance accompanying the lot, the coin's owner said: "My granddad had travelled all over the world during his working life and had collected many coins from the various countries he had been.
"He gave me bags of coins to play with (I was into pirate treasure) throughout my early years.
"As time passed these coins went back into bags and boxes and were forgotten about until I re-discovered them after my granddad passed away.
"My little boy has been playing with this coin as I did all those years ago," he said.
The coin was made 314 years ago and was the first British coin ever to feature the name of the battle it was won in.
The series of Queen Anne 'Vigo' five guinea pieces were produced as part of an attempt to detract attention from the failure of the British to capture the Spanish port of Cadiz, highlighting instead the haul of treasure they seized on their way home.
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