It does look like it’s darkened a little and gotten larger since the thread started. Not good, not good at all. Nice catch @Muzyck.
The mark is either some dirt or corrosion of some sort that looks like an s. That's all there is to it. There is no way the mint could do something like that.
No way for this to be a mint error. It's a stain, corrosion or any number of other things including ones imagination. Our minds plays tricks and we see what we want to see, not what's really there. I can see what looks like an "s" but it's not a mint error and only worth face. Welcome to CT.