Mintmark S or just damage ?? I no the Mintmark is not supposed to be on the reverse for this year but it sure looks like one
You're seeing what you want to see. Think about how mint marks were made on dies in this era. How would one have shown up on the reverse of a coin?
The mint mark is under the date. It's a 1920 Philly which were produced in high numbers. There's nothing on the reverse. If you see something, it's some kind of mark, scratch, damage.