The collection, built up by Mac McCarthy from Cornwall, fetches £21,830 at an auction in London.
Fewer than 300 of the gold "Una and the Lion" coins were produced, with one found in a Bangor estate.
A total of 213 coins discovered in a cottage in Dorset are expected to reach £30,000 in auction.
Clinton Bowen disappeared during his trial for the theft of £500,000 of rare coins from a dealer.
The 5p, bearing the King’s head and an oak leaf, follows the earlier release of new £1 and 50p coins.
The Oaks Post Office was among other branches chosen because they either have "Oak" in their name or are close to famous oak trees.
David Moss discovered 15,000 coins at a "virtually untouched" secret location in north Wales.
The couple found a lump of "strange circular discs" while adjusting fence posts in their garden.
A solid gold version of the coin will also be hidden in a limited edition advent calendar.
