20p coin had to be scrapped over similarities with existing tuppenny
Plans to introduce a distinctive new 20p coin were shelved after it left consumers short-changed - a blunder which has turned out to be lucrative for some.
In 1985 the Central Bank's original design for a fiche pingin was tested in old phone boxes, parking meters, bus conductors and coin counters and found to be too easily confused with the tuppenny already in circulation.
State papers from the Department of the Taoiseach revealed a batch was produced by the Royal Mint in the UK and delivered at the end of February that year but introduction was pulled four months later after the tests were failed.
About 500 coins were sent to CIE, Telecom Eireann, Dublin Corporation and some businesses, most of which were returned and melted down.
The documents, released under the 30 year rule, stated: "Serious difficulties in distinguishing the new 20p from the existing 2p were discovered.
"The specifications of the proposed coin were published in August, 1984 but the extent of the difficulties did not become clear until May/June 1985."
Designers set about reconfiguring the coin, albeit a year down the line, with a barely visible stretch of the diameter from 2.6cm to 2.71cm, "making the piece marginally heavier".
But the blunder has turned out to lucrative for anyone who managed to pocket one of the 50 or so coins which were originally minted but never made it back to the Central Bank to be destroyed.
One coin believed to be from the rare batch sold for 7,200 euro in an auction last month and it is thought others remain in private hands undetected.
The new coin order, agreed after the redesign, said 20ps were to be made of an alloy of copper, zinc and nickel, giving them their distinctive yellow-brass hue and weighing 8.47 grams.
It featured an Irish hunter horse that was on the pre-decimal half crown in circulation from 1928 to 1967.