Dear Ms. Roberts,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located 144 file units in the Records of the U.S. Marine Corps (Record Group 127) about the 51st Marine Defense Battalion during World War II. For access to and/or copies, please contact the National Archives at College Park - Textual Reference (RDT2) via email at firstname.lastname@example.org.
Also we located the series General Photograph File of the U.S. Marine Corps, 1927 - 1981 in Record Group 127 that included a photograph of a gun named ‘Lena Horne’ surrounded by veteran crew of 51st Marine Defense Battalion; and the series Negro Activities in Industry, Government, and the Armed Forces, 1941 - 1945 in the Records of the Office of War Information (Record Group 208), which includes a photograph of the first black man to enlist Howard P. Perry. These items are digitized and available via the Catalog.
Additional photographs of various Marine Corps activities dating from 1940 to 2007 are in the custody of the National Archives at College Park - Still Picture (RDSS). Please contact RDSS via email at email@example.com and their web site is https://www.archives.gov/dc-metro/college-park/photographs-dc.html.
We suggest that you search Ancestry U,S. Marine Corps Muster Rolls, 1798-1958 if you have the names of those African American marines who enlisted in the 51st. There may be a fee for using Ancestry. Instead, you may view these records online at one NARA's facilities for free via a NARA PC. For the nearest NARA location, please consult our web page at https://www.archives.gov/locations/. Also, some of the images from Ancestry are available for free on Family Search at https://www.familysearch.org/en/.
You also may wish to review Marine defense battalions; African-American Artillery Defense Battalions in World War II (US National Park Service); and The Right to Fight: African-American Marines in World War II (Building the 51st Defense Battalion) websites for additional information about the 51st Marine Defense Battalion.
We hope this is helpful. Best of luck with your research!