A friend of mine got a number of these buttplates. At first glance they have the look and fit of M1903s but as you can obviously see, they have no "trapdoors". Although a similar shape, they also don't fit over a M1903 or M1917 (the shape for a M1903 is similar, but the screw holes don't line up. Obviously not M1 or M14.
They have a charcoal-type finish, like you see on late WWII parts.
Any clues?