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I found this coin and have been trying to figure out what it really is that caused the "shelf" appearance.
If you're referring to the spot on the inside of the curled part of the 6, right where it stops at the top of the curl, that's actually on all 1960 small dates and is one of the key identifying criteria of the small date versus the large date. It's my understanding that it was engraved onto the master hub for the small date and thus on all working dies.
If you look at the obverse design variety page on Variety Vista you'll find it designated ODV (Obverse Design Variety) 22, which means that little notch or shelf like spot is on all of them and thus not MD or doubling.