The coin was brought in to the firm's Tunbridge Wells office along with a number of other more run-of-the-mill pieces by a longstanding client. Director Leslie Gillham immediately recognised it but brought in a second opinion to make sure that it really was as good as it seemed to be. Clearly it was.
"What is special about this rare example is that it was struck from bullion captured at the Battle of Vigo Bay against the Spanish," said ATG's coins correspondent Richard Falkiner.
Mr Falkiner added that he and others had "raised their eyebrows" at what they deemed a stiff £80,000-120,000 estimate, but were delighted to be proved wrong when the hammer fell to Knightsbridge Coins at double the top end.
"This coin although very rare does occasionally turn up," he said. "Spink sold one in 2005 for £130,000 and they got a slightly inferior example away at £82,000 two years later."
The sale took placeon December 6.
Spink were also to the fore on December 5 with the gold coin discovered in August 2012 at Cleybrooke Magna in Leicestershire, only a dozen miles from the site of the Battle of Bosworth. The Angel, dating to c.1484, and bearing Richard III's personal emblem of the boar, features St Michael spearing the dragon on reverse and a ship on the waves with a crucifix above a shield on the obverse.
Its excellent condition indicates that it was lost when freshly minted, and it was described in Spink's catalogue as 'essentially uncirculated'.
How was it estimated? Spink's 2012 annual catalogue of British coins lists this coin at a full retail price of £27,500. However, this example was in even better condition than the 'very fine' specified. There was also the fact that the timing of this sale coincided with the recent identification of what are thought to be Richard III's remains.
Spink opted for a modest but tempting estimate of £12,000-15,000 but it trebled the low end of that to take £36,000, selling to a private collector.
Specialist William MacKay said: "There was fierce competition in the saleroom for this exceptional example of an historically important coin from an interesting find location. The result shows the demand continues to be strong for top quality, rare English gold coins."
Meanwhile, Salisbury's Woolley and Wallis have made another valuable coin find, this time among the collection of a boy who died, aged just 20, as the result of a mountaineering accident in 1960.
It is a very rare United States 1796 copper half-cent, of which only 1400 were struck, and is in superb condition. It will be offered on January 22 with an estimate of £25,000-30,000.