Is that a weld around the edge. It looks like it in the one picture but it might be dirt. I don't know why they would box it either unless for strength. Why would there be a counterweight under the engine? An interesting puzzle for sure.
This is surely a fabricated piece. There are a couple are a couple pretty good pics on Machinery Trader and it looks to me that it's only held up there by 6-8 bolts. As OIH said it doesn't make sense for the counter weight to be there anyway. Just my $.02