Williamson, H. F., & Smalley, O. A. (1957). Northwestern Mutual Life: A century of trusteeship. Northwestern University Press.
शिकागो शैली (17वां संस्करण) प्रशस्ति पत्रWilliamson, Harold Francis, और Orange A. Smalley. Northwestern Mutual Life: A Century of Trusteeship. Evanston, Ill: Northwestern University Press, 1957.
एमएलए (9वां संस्करण) प्रशस्ति पत्रWilliamson, Harold Francis, और Orange A. Smalley. Northwestern Mutual Life: A Century of Trusteeship. Northwestern University Press, 1957.
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