just got this beauty on mail today , first time I saw it I can't move my eyes off it ,So beautiful in the hand.
US! It was an original Yen from 1951, it traveled across the Pacific Ocean to the USA and struck in 1972 at the San Francisco Mint! Most likely with the help of a bored Mint employee who tossed the Yen into the 25C Planchet bin!
It's not possible for that to have been an "error". It was a deliberate action. Every Proof coin is visually inspected at several points and would have been caught at some point if it was a true acident. This had to have been made by the press operator who put the Yen in the feeder slot and then pocketed the result as soon as it left the press.
It was an error for the US mint to let it out that way. Obviously meant to be done by the one who did it.
I don't know what happened but it was only know piece exist. There are NO Others Known, The ONLY1972-S PROOF Washington Quarter Struck OVER a Japanese 10 Yen coin. Certified by NGC
Wait!... there might be another. According to the now defunct MintErrorNews.com website there was one certified by PCGS! 1972-S PROOF 25c struck on an already Japanese 10 Yen. A proof double denomination on a foreign struck coin, Only 1 known PCGS. The US has never officially minted any coins for Japan.
I think pcgs count the one from NGC that thy say only one know. If pcgs have other one it will say only two know.
I would not call it an error either. It was a deliberate attempt to mislead. I have a British 10 Pence struck on a 5 Pence planchet, but perfectly centered. It had to been placed on the die by someone at the Royal Mint