Plate thing?

[bblaha]

back up on top of a hill 8 inches in the ground I found the plate thing, it looked to be brass at first but now looks aluminum, it has a "49" stamped in the top right corner, an R just left of center, and a G just right of center. There are 3 small holes around an indent in the center and a punch out on one side. Any info would be appreciated thanks HH