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You can find information directly related to the United States Navy Seabees via their website at https://www.history.navy.mil/content/history/museums/seabee.html
You can find individual histories of specific Seabee Units here: https://www.history.navy.mil/content/history/museums/seabee/explore/seabee-unit-histories.html
The material from the Seabee Museum may assist you in locating exactly where your Grandfather served. The National Archives at College Park, MD, houses U.S. Navy muster rolls from approximately 1940 to the early 1980s.World War II–era Navy muster rolls are available online through Ancestry.com (database title: "U.S. World War II Navy Muster Rolls, 1938–1949"). Please note that Ancestr.com is a fee based web service. Ancestry.com may be available at no charge at your local library.
For more information about these records, see Know Your Records: U.S. Navy Muster Rolls and Personnel Diaries. For questions, please email email@example.com.
It appears you have already searched for your Grandfather's Service record however, if you’re trying to find more information about service records you may want to request his Official Military Personnel Files from the National Personnel Records Center. Note that due to the COVID-19 pandemic, the NPRC has been closed and is only completing emergency requests. As of March 8, 2021, the NPRC entered into a phased expansion of our onsite workforce. They are continuing to increase their on-site staffing and are servicing requests associated with medical treatments, burials, and homeless veterans seeking admittance to a homeless shelter. For that reason you want to refrain from submitting non-emergency requests until they return to pre-COVID staffing levels. You can check archives.gov/veterans for updates to the NPRC operating hours and status. We apologize for any inconvenience.
We hope this information helps you get started with your research. Best of luck!