Like so many other scientists, J.-B. Colbert de Beaulieu made corresponding with other scholars an important part of his scientific practice. In the early 1990s, he donated a prodigious collection, containing over 5,000 letters dating from 1948 to 1992 to the Department of Manuscripts of the National Library of France. These unpublished archives are of interest as a testimony of the workshop of this scientist. We explore them by selecting a specific case, viz. the Traité de numismatique celtique. In other words, the correspondence of J.-B. Colbert de Beaulieu is used here as a means of ‘retrospective intelligibility’ to enlighten us on a project that is a landmark in the history of our studies. Thence, and by going with caution through the correspondence, it is possible to make original contributions to the understanding of the scholar and his work.