I am not an expert on Seated quarters or Carson City coinage but after looking at some certified Seated quarters on Heritage, there is a difference in the styles of the CC mintmark. I did a quick look and it seems that at least some if not all the quarters from 1870-1873, 1875 used a mintmark where the C's are a bit smaller and rounder than the larger more oval shaped ones that I saw on those from 1876-1878. It does not seem to be a unique thing as other series like Seated half dollars and morgans also had different CC mintmark styles. Below is a link to a online book on Seated half dollars that shows different varieties of O,S, and CC mintmarks found in that series.
http://blog.davidlawrence.com/index...tmark-sizes/
http://blog.davidlawrence.com/index...tmark-sizes/



















