1960D small dates are far more common than 1960-P small dates. They also did have quite a bit of chipping, so that is also what we are seeing here. Keep up the hunt!
what i am thinking though is that maybe someones collection was stolen. i have never seen the same exact error multiple times in the same grade. i am half way through my 2nd of 4 boxes and found 2 more. so if someone stole a collection and put it in the coin machine could it happen i will get them in a lot?
Not sure who would have a whole collection of only 1960-D small dates with die chips, as they aren't really worth much, but I guess it's a possible explanation.