Dear Mr. Silverman,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located General Decimal Correspondence Files, 1942 - 1948 in the Records of Headquarters Army Ground Forces (Record Group 337) that includes a file unit 300.3 AR 615-26 [Army Regulations]. This record has not been digitized yet. For more information, please contact the National Archives at College Park - Textual Reference (RDT2) via email at Archives2reference@nara.gov.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from RDT2. We apologize for this inconvenience and appreciate your understanding and patience.
We hope this information is helpful. Best of luck with your research!