Just took a look at the images. Not a lot of detail for sure. But I'd like someone else to look and see if they see what I think I see! Hard to tell, but that extra plate looks like another Left side to me. Maybe my eyes are out of focus today.
MG08 pretty well summed it up on 80% plates. When those were being made, the specs varied with the manufacturer. ATF generally ignored all this for years, but what they settled on, after closing the Halo case, was the charging handle slot and feedway profiles. They don't have any concern about what percentage of the plate is finished. That is not their criteria, and they refer to that as an "industry term." Their concern is how much time and what kind of tools it requires to be finished. Not that this isn't equally vague, but the major machining that is generally considered to require a mill, is a lot different than just a drilling op. All these discussions we used to have, like once you drill that one extra hole on your 80% plate you now have a gun, has proven to be meaningless.
Note, on material: Drawing calls for these options: A331, A322, 4340 or 4140. Heat treat of last inch and a half, to 32 Rockwell "C" scale. The .30 cal guns didn't get heat treat added until 1942. If that is the same pattern for the 50s, then the heat treat was added later on, after many of these were built and issued with no heat treat. I don't have drawings early enough to confirm that on the .50 cal, but there is enough to indicate this was a revision from original specs.