1990 Lincoln

Anomolous little bumps or die scratches can often make you see things that aren't really there if you want it bad enough. Unfortunately I believe this is a case of just that. I see what you're pointing to and can "sort of" make something out there, but what you're showing as an S has the wrong shape and size to be a mintmark. Looks like either a series of tiny gas bubbles, a die scratch, or just the way the dirt toned to look like something.