Aquachigger, to me the stamping looks like a C, or perhaps a G, but it could also be a Q. Looks like some encrustation is obscuring the lower right side of the stamping's area. Can't be certain about what the letter is until the encrustation gets removed.
I think the mark is highly unlikely to be a G, because in 36 years of observing and researching such markings on projectiles, I've never seen (or heard) of a Read Long-Model projectile with a G on it.
The stamping on your shell is in an unusual place. I've never seen a "solitary" letter stamp located on the lower half of a Read Long-Model projectile.
Please tell us the name of the state the shell was found at. Also, do you have a guess about the approximate date during the war that your shell wound up at where you found it?
Dave is right, if that shell was found in saltwater or even "brackish" water, it needs Electrolysis treatment to prevent cracking. But if it is definitely from freshwater, Electolysis may not be required ...and might even do harm to the shell. Many shells from the freshwater river at Augusta GA and Milledgeville GA have held up fine (for 20 years) without Electrolysis. That being said... some of them were "Graphitized," and therefore could not withstand Electrolysis without losing some of their original iron surface. (Graphitized iron is verry soft, and will "come off" in Electrolysis.)
Regards,
Pete