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DOS 2 was the name used to refer to a Salyut module. Launch by a Proton rocket was attempted July 29, 1972, but a second stage failure prevented the intended space station from reaching orbit. Instead it fell into the Pacific Ocean.
References
Portree, David S. F. (1995). "Mir Hardware Heritage". NASA. http://ston.jsc.nasa.gov/collections/TRS/_techrep/RP1357.pdf.