The active correspondence between Auguste Tolbecque (1830-1919) and Victor-Charles Mahillon (1841-1924) preserved at the MIM of Brussels is an interesting source for the study of musical instruments reconstructions at the end of the nineteenth century. In 1879, Tolbecque sold to the Instrumental Museum of the Royal Conservatory of Brussels a large collection of ancient instruments to cover the upkeep costs of his new home in Niort, the Fort Foucault, which he had acquired in 1875. It was undoubtedly at this sale that Mahillon and Tolbecque met. Having put his career as a solo cellist and composer on hold, Tolbecque devoted his time and research to instruments making, creating around sixty reconstructions from Antiquity to the Renaissance. His learning in violin making with Claude Victor Rambaux (1806-1871), alongside his years of musical studies at the Paris Conservatory (1845-1849), enabled him to envisage making these “copies”. His famous “panoplies” of “instruments disparus [disappeared instruments]” were presented at national and universal exhibitions between 1889 and 1896. The correspondence contains information relating to the preparation of the various series of reconstructions. What was the content of these letters, views and the differences between Tolbecque and Mahillon in their respective approaches to the subject? In his search for models for his museum, Mahillon seems to have favored commissioning instruments inspired by Antiquity and the Middle Age from Tolbecque — the most problematic to design at the time, both in the choice of historical and musical sources preserved and by the technical knowledge to be implemented. While Tolbecque’s instruments relating to Antiquity have been the subject of particular study in recent years, those relating to the Middle Age — to which we can add a few reconstructions from the Renaissance — provide just as much scope for interesting epistolary exchanges between the two scholars. Methodological reflections, sources (books and iconography studied), techniques (sketches of instruments, materials used) are at the heart of the discussions between Mahillon and Tolbecque and offer a first-rate account. Although secondary according to Tolbecque, the financial question also appears throughout the exchanges. Finally, the content of these letters can be compared with another series of reconstructions: that sold by Tolbecque to the banker and collector Charles Petit of Blois in 1896, with whom he also kept up a brief but interesting correspondence. As conclusion, we will be able to give a first view of all reconstructions made by Tolbecque between 1865 to 1899 and currently held in three Europeans collections.