I posted this once before don't think anyone responded at first I think extra thickness which I still think it is ddo 1 but look at United @Rick Stachowski
http://www.coppercoins.com/lincoln/diestate.php?date=1978&die_id=1978d1do001&die_state=mds Isn't that matching
Here's coppercoins image . Here's your image . The placement of the " D " on coppercoins, is closer to the number 9 ...