Can you post a picture of the full obverse and the full reverse of the coin?
I've never heard of a large "S" vs a small "s"mint mark on a 21-S
Morgan dollar, either. Every 21-S Morgan I've ever encountered had a small s. It is one of those coins that once you see the reverse there is no need to flip over the coin to check the date. You KNOW exactly what the date is.
I'm guessing your pictured coin is counterfeit. It just doesn't look "right" to me.