The Linde Air Products Company and Electro Metallurgical plant near Niagara Falls built the ring-and-plug in Little Boy.
Linde Air used the Lake Ontario Ordnance Works site at the end of the war to dispose of its atomic waste, cuttings from the African uranium, some 200 dump trucks' worth.
It also held very high concentrations of thorium and radium (and their decay products, including radon gas) which are retained in the tailings (residues).
The very high concentrations of these extremely toxic, long-lived radionuclides present in these wastes prompted the National Academy of Sciences' National Research Council to categorize them as indistinguishable in hazard from High-Level Waste in its 1995 report, "Safety of the High-Level Uranium Ore Residues at the Niagara Falls Storage Site, Lewiston, New York" [1].
The Mallinckrodt "K-65 residues" were later moved to the Feed Materials Production Center, a Cold War era uranium refinery at Fernald, OH (outside of Cincinnati) which commenced operations in 1951.