does the other side look the same of does it have any detail? It might just be some lead that was melted into a can or tin cup for some reason. I found some pieces like that and boy they can get you going.
I'm thinking that it "could" be, especially considering how bad the other belt plates were eaten up from the soil composition...
but it could also have the possibility of being something else.
Some of the US and CS plates had lead fill in the back to hold the brass piece that went in the leather. This piece of lead is also tapered around the edge where the rim of the plate would be. Thats what makes me think it is part of a plate. Good find buddy, I bet you're ready to go back out there. I know I am!
peanut
It could be the back of a plate. I'm not sure on this one. I've seen ground so bad that the brass face will be completely eaten away and just the lead is left. The ones I've seen have always had the detail from the face on the lead though. buckles that have come from shipwrecks are the same way. Either way it's a big piece of lead and if that was there who knows what else is left. Tear that place up man!!!
It is round, not oval. It has the raised rim around the edge, and it seems to be roughly the size of an eagle breast plate (which also has rhe raised rim and wire fasteners).