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Dear Ms. Collins,
Thank you for posting your request to History Hub!
Army Regulation No. 615-30 dated February 12, 1942 has an explanation of the Army Serial numbers for enlisted personnel and lists a few of the letter prefixes in the changes to the regulation. Correspondence in the Legislative and Policy Precedent Files, 1943-1976 under File 593-121 contains even more listings of the prefixes. For access to these documents, please contact the National Archives at College Park - Textual Reference via email at email@example.com.
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