M121 155mm Cartridge
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The U.S. Army standardized the M121 shell in 1954.
See also 
- Smart, Jeffery K. Medical Aspects of Chemical and Biological Warfare: Chapter 2 - History of Chemical and Biological Warfare: An American Perspective, (PDF: p. 51), Borden Institute, Textbooks of Military Medicine, PDF via Maxwell-Gunter Air Force Base, accessed November 13, 2008.