Actually, such markings are an Inspector's mark. The Inspector may represent a government arsenal ...or a privately-owned Metal-Finishing shop, doing subcontracted work for a government facility.
I've been able to deduce the particular arsenal-connection of some of the letter-marks (such as, G represents the Selma Alabama aresenal's inspector). I've seen the T before, on 10-inch shells, but I don't yet know who it represents. It is a very rare marking.
It'd help my research to know what general area you dug the T-marked 10-inchers at. That's because marked projectiles tend to be used most frequently in the same general area as the manufacturer who made them was located. For example, D-marked projectiles are found most frequently in the Atlanta area.
Also, have you dug some with any other letter-mark?
Send me the info in a Private Message if you prefer confidentiality. I will not betray your Trust.
Regards,
TheCannonballGuy (PCGeorge)