It is from sometime in the 20th-Century. Are you sure the backmark doesn't actually say "COS" instead of "CO"? It it says CO'S it is from 1943 and later.
Post civil war. worn by State officials like police,etc. A good clear picture of the back-mark might help id the time frame. CannonBallGuy should be able to date it for you i.e. http://www.treasurenet.com/forums/what/361541-need-id-button.html will tell you some info on dating WaterBury back-marks. It could date from 1880s-early 20th century.