Well, I was hoping to find out what this was before posting, but haven’t found anyone who knows. So far, the guesses are: 1) a license plate, but I asked my neighbor that restores old cars and he said it wasn’t one he’s seen, or 2) a grave marker… but I don’t feel confident on that either.
It is metal, of course, with a thick glass which used to be there in the middle. There were small pieces of paper under parts of the glass, but they basically disintegrated as I took it out of the ground. There are no signs this was hooked to something else or part of something else. There is one long hinge at the top and two small metal flaps, at the bottom in the back, that close the two plates together over whatever paper was inside.