What Did this Mirror come off of

That is a Mopar a-body painted sport mirror, it was an option on 71-76 Duster, Demon, and Dart Sport models, also made them in chrome. Very nice of him to give that to you, tough o find and worth over $100! It will fit other a-body cars as well, the driver side mirror is usually a little easier to find and cheaper too. I would start a "parts wanted" ad, I am sure somebody has a left mirror. Also, check the base for a part number to see if it's a factory mirror, there are repops out there too.

BTW, is that a new old stock one or used? It looks really nice, I had a NOS right one for a 70 e-body, it was primered like that one too.