I dont know if the pictures are clear enough but it looks like an S under the O there is definitely something there and im wondering what it is. If anybody can tell thanks.
It gets too blurry when I enlarge. The only over date for a 1891 Dime is the O over a horizontal O. An O over an S would not be impossible. I can not be convinced that an O over an S would be discovered after 128 years.
1891 o /o medium o over medium o variety 106 http://www.seateddimevarieties.com/date_mintmark/1891o_106page.htm
