Errors like this are not uncommon on
Severan era
denarii, and later into the third century.
RIC published some known common errors, but the book came out in 1936 and has not been revised since then. It doesn't list this error variant, but that also is not unusual.
It's likely most
Roman plebs never read these inscriptions, assuming they were even literate, but the
snake motif would immediately have been obvious to them, pointing to the depicted deity being
Salus.
Nice coin.