i have a perf 10 vert 15cent Ft. McHenry Flag and every website I find says different SC#. SC info is not very clear. I cant tell if it is 1597, 1598, 1618c. My scanner is down so I cant provide picture but this probably doesn't need one.
If it is a coil (perforated on two opposite sides) the only choice is 1618C. If your stamp is perforated on two non-opposing sides (top/left, bottom/right etc.) or three sides you have a 1598 which would have different perforation rates anyway.