Dear Chris Shields,
Thank you for posting your request on History Hub!
The US Citizenship and Immigration Services (USCIS) maintains duplicate copies of court records (including the certificate of citizenship) created September 27, 1906-March 31, 1956 within Certificate Files (C-Files). Beginning April 1, 1956, INS began filing all naturalization records in a subject’s Alien File (A-File). If you cannot locate your grandfather’s naturalization date within his records, we suggest that you submit a Name Index Search to the US Citizenship and Immigration Service (USCIS). The USCIS Genealogy Program will send you a letter reporting the search results that will identify all file references found in the index and provide instructions and additional information necessary to request the file(s) from the USCIS Genealogy Program or the Freedom of Information (FOIA) Program.
Since you are pursuing dual citizenship, the Consulates are aware that USCIS does not provide certification of files in their holdings. Instead, you must present the photocopy of the naturalization certificate along with the USCIS Genealogy Program response letter and mailing envelope. We suggest that you consult with the country’s Foreign Consular Office for more information.
We hope this is helpful. Best of luck with your family research!