Ten rare Isle of Man 10-pence pieces, thought to be the only mint examples left in existence, will go on auction next month and are expecting to fetch at least £400-500.

The 1992 10p coins were issued to replace the larger old 10p, mirroring the new UK version in size, weight and design but bearing the three-legged Manx triskelion instead of the lion.

Pobjoy Mint is understood to have released a pre-production run of 200 coins bearing an ’AA’ die mark for retail businesses and vending operators to test.

But while machines could accept the new UK 10p, the Manx coins were rejected and the mint asked for them back.

Shortly before the recall, a young man opportunistically purchased 10 of these pre-production Manx coins from the treasury counter.

As well as the initial AA die mark - the post-recall reissue of the Manx 10p bore a mix of different die marks such as AB and AC - the ’PM’ mint marking is also missing from the pre-production coins, which are sealed in individual protective packaging embossed with ’Pobjoy Mint’.

The anonymous vendor, who lives in the island, said: ’The packaging is the biggest anomaly for a coin with such a low face value.

’Each coin is individually sealed in its own plastic bubble within a sheet, which is not how you would normally expect coins of this denomination to be shipped - they are normally lumped together in £5 bags.

’It would be cost prohibitive to package every 10p in its own individual wrapper as many thousands of coins would have been produced in the main production run to replace the larger 10p coins.

’I have noticed they are all missing the PM mint mark which is present on other 10p coins, usually appearing directly below the Queen’s head. Could this also have been another reason for the recall of the pre-production run to the Pobjoy Mint?

’Information is not easy to come by and, as the coins have such a low face value, no-one really paid a lot of attention to them back in 1992 - the most appealing thing about them at the time was that they weighed significantly less than their predecessor.’

He added: ’I do occasionally get a 1992 10p coin in my change, I check the die marks but I have never had a AA die mark coin from 1992.’

The coins will go under the hammer at the The Lichfield Auction Centre in Staffordshire on Tynwald Day morning.