The name of Antedios is inscribed in full with a Greek letter, which is unusual as abbreviations were more common.
Mike, 63, a storeman from Newport, who found the coin in January while out detecting with his brother Tony, said: “I was detecting on farmland where my brother and I have a permission.
“It was a wet, muddy day and the field had been recently ploughed. I got an iffy signal but decided to investigate and after digging down four inches, there it was.
“I got down on my knees in the mud and picked up the coin. I studied it and was absolutely amazed and dumbfounded.
“I did a jig of joy and put it in my brother’s hand and I can’t repeat what he said! I’ve been detecting for 14 years and it is the best thing I’ve ever found.
“I’m going to split the proceeds with the landowner and my brother so whatever is left I’ll spend on a holiday.”
The “extremely fine” stater, which is under an inch in diameter, is going under the hammer at Timeline Auctions, of Harwich, Essex.