Perhaps this could be an unlisted RPM? There appears to be the rest of the S in the interior of the D. Admittedly, that could just be a die gouge, but I'm ever the optimist. At the end of the day It may be a trick of the brain (as oft coin errors are), but I'd like to ponder such a convincing illusion!
OP (CoinHobbyist) Could you please post pictures of the entire coin?