yeah, the "I" for infantry is visible. if it were plain it would be a general service button. "C" for cavalry, "A" for artillery, "D" for dragoons, "R" for riflemen (there is an eagle "H" button out there but I will skip that one as it likely was intended for a specific unit). often backmarks are important in dating buttons but in your case the front is enough to tell me it looks like a good period button; meaning pre-1865. The "I" on yours is a block type font. Sometimes the 'split-top" "I" button is postwar.