The "1951" date refers to the year that the Krewe of Hyacinthians was formed, not the year they were minted. These generally sell for a slight premium over spot. I've owned quite a few silver Mardi Gras doubloons, and most of them weighed just under 1 troy ounce. Your best options for selling them are either ebay or your local coin shop. Or call Rick Demers at The Coins & Collectibles Shop in Kenner.
As for the aluminum ones, they are mostly worthless except for very early ones and some of the Krewes (such as Zulu). Ones designed by H. Alvin Sharp are keepers - they usually have HAS somewhere in the design. Even these are only worth a buck or three.
Inordinately fascinated by bits of metal with strange markings and figures























