Dear Mr. Gordon,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located Signal Unit Records, ca. 1965 - ca. 1973 in the Records of the U.S. Forces in Southeast Asia (Record Group 472) that includes Organizational Histories of the 521st Signal Company from 1945 through 1973 in Box 362A. For access to these records, please contact the National Archives at College Park - Textual Reference (RDT2) via email at email@example.com.
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We were unable to locate after action reports of the 521st or the 11th Signal Brigade for 1975. Please provide an image of the first page of the report you seek in order for us to assist you better.
We hope this is helpful. Best of luck with your research!