Unknown trapdoor

[.69caliber]

Hello,
I dug this in one of Kilpatrick's 3rd Cavalry Division camps here in Lovejoy. Does anyone have an idea as to what it went to. It's brass and beveled all the way around the inside. I would guess that it's for a rifle or carbine butt plate or patchbox. I haven't been able to match it up with any firearm I'm familiar with. Found with a lot of Sharps, Spencer, Colt RR, Burnside and Smith bullets.
Thanks for any help.

[TnCobra]

H. Leman patch lock.

These showed up on early percussion and converted to percussion fintlock rifles. Most likely of the Kentucky style rifle.