So another 28-S for the hoardpile showed up today, I opened the envelope, looked it over, and before I threw it in the Small S roll, noticed that it has what looks like a die chip on the reverse. I have not seen this before and it's not in the Knauss book. Right wing of Mr Eagle, extending from about the 4/5th feather from the top, up to and including the "S" in "PLVRIBVS" Kind of neat, eh?
Turns out it's a die clash. Threw me off because you normally see evidence of them on the obverse, but it is indeed a clash.