It is not a die variety; it is a die state. A die variety has features that were part of the die’s original design. A die state results from die wear while in use. Die state markers include die breaks, die cuds and fading of the details from General die senility. What you have here is a die state or perhaps an error from grease that filled the die’s features. Remember that everything that is raised on the coin is into the face of the die.
Just wanted to follow up on this one. I found another 1951 thin 1 (see below). It's not quite as thin as in the original find, but it made me wonder. I found a couple other thin 1's online after that. 1951S-1MM-018 and 1951S-1MM-20 on the coppercoins website both have thin 1's. So it looks like the thin 1's are part of variety(s) and the S is likely from grease fill. Thanks all.
