Possibly the top to a glass-bowl fuel filter?
edit - maybe that's not right, as most I see are aluminum (so they don't rust and contaminate the fuel).
There may be a word or numbers along the right side edge of the flat side? Cleaning off a bit of the rust/scale might make an ID easy if there were markings.
thanks good guess, it does resemble one in some ways, but i'm not convinced yet i will do a bit of cleaning on it and look for any markings, sure looks like the middle part of a projectile that held in the explosives but thats a total guess would like to know what this really is. thanks for the input nhbenz.
sort of reminds me of the plate that holds the coil of a hot water heater coil that gets bolted to a hot water furnace to supply hot water to the sink.
Most I have seen have a few more bolt holes to hold it onto the furnace, but maybe because it is smaller, the one my grandfather had on his furnace the iron plate was maybe 8" in diameter.