Can only think of a couple: Silver countermark by Tokyo Mint. I specialize more in overstruck coins, not countermarked.
Here's one of my most recent pickups Derick. Can't tell what coin it's struck on but looks early U.S. colonial. This one's from the 1850's. Bruce
The coin's copper Derick. I forget the size and weight of it off hand, but it's about the size of a U.S. Large Cent. The man I bought it from is a well known oldtime collector and he couldn't determine the coin type either. Bruce
Here's two more counterstamps...on modern coins. These were issued by the Western Reserve Numismatic Club of Cleveland, Ohio to celebrate their anniversary. Bruce
The top one cat no. KM 163. 1923 Costa Rica Colon counterstamp on 1880 GW 50 centavos KM 112. It seems to be 12.5 g and 0.75 Silver
Derick, here's more info of the Costa Rica counterstamp. http://www.coinfactswiki.com/wiki/Costa_Rica_1923/1892_50_centimos Now that I remember, if mintmark counterstamp counts, here's a few more: I actually did hit the press in the Canberra mint to that counterstamp. 9th Feburary 2007, 11:58am. - Wow that's quite a while ago. A NCLT coin. This one managed to escape into circulation somehow - it's a NCLT coin but with a counterstamp.
