Well... actually BuckleBoy, a lot of my mid-1800s-to-early-1900s "entangled letters" button identifications come from knowing that many are Schools-&-Universities buttons, or "local" (city or county) Militia buttons. Therefore, when a final letter of 2 or 3 letters is:
A is often for Academy
C it is often for College
I is often for Institute
S is often for School
U is often for University
G is often for Guard or Guards
SS is often Steam Ship
L is often Line (or Lines) such as Cunard Lines
and of course R (or RR) is often for Railway or Railroad.
Also, if you see a C, look for a smaller letter O -- which with the C forms the abbreviation for Company. Check the image below, which is what the Waterbury Button Company catalog calls a "Career Uniform" button... for the Union Traction Company.