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Albert Wohlstetter, "Nuclear Triggers and Safety Catches, the 'FSU' and the 'FSRs'" (1992), in Robert Zarate and Henry Sokolski, eds., Nuclear Heuristics: Selected Writing of Albert and Roberta Wohlstetter (Strategic Studies Institute, 2009), pp. 374-377. Originally in: Unpublished note, February 6, 1992, available from the Hoover Institution Archives, Albert and Roberta Wohlstetter Papers, Notes, Box 121, Folder 1. Courtesy of the Wohlstetter Estate.