It is the serrated "plunger" (also called detonator) from a civil war Confederate artillery fuze called a Girardey percussion (impact-detonation) fuze, named after its inventor, Isador P. Girardey. Even though it's only a part of the Girardey fuze, it is the most difficult part to find. It MUST be from an unused fuze, because the Girardey fuze gets destroyed by the shell's impact with the target and resulting explosion. As you would guess, the Confederates tended not to throw away an artillery fuze, unused... and especially, not a Girardey Percussion fuze. I must say, Congratulations on finding an extraordinarily rare Confederate artillery relic.
By the way, that's not a letter C on the flat disc. It's where the detonator's serrated blade connects to/through the disc.
Girardey Percussion fuzes are known to have been used in the Atlanta Campaign, and several have been found in the Confederate artillery emplacements and trenches near Kennesaw Mountain.
I advise you to hunt the spot where you found it very diligently. The rest of the fuze, or more of them, may be there. Here are some photos, and a "cutaway" diagram of a Girardey fuze in a screw-in fuze adapter plug from my book on civil war artillery shells, to help you know what to look for.
