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Dear Mr. Comer,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located one file unit & 6 items that pertain to Virginia Hall from 3 series in the Records of the Office of Strategic Services (Record Group 226). The items are digitized and available for viewing via the Catalog. The files may include an explanation of her spying techniques.
For information about Marie-Madeleine Fourcade and her spy network, we suggest that you contact the Directeur des Archives de France, 60 rue des Francs-Bourgeois, 75141 Paris, France. The web site is https://www.archivesnationales.culture.gouv.fr/.
We hope this is helpful. Best of luck with your research!