This paper aims to draw attention to one of Silvain Pouvreau's manuscripts, This document to which not proper consideration has been given to, is part of the Fonds Celtiques et Basques of the Bibliothèque Nationale de Paris (BNP 8, fol. 201-206). We publish here this manuscript (we've called it C manuscript), where Pouvreau copies, completes and remakes the best known one till now (the A manuscript, BNP 7) demonstrating that Pouvreau's purpose was the Basque-French-Spanish-Latin Cuatrilingual Dictionary. We'll try to show that according to the several notes sent by Oihenart to Pouvreau and included here, this C manuscript was written in 1665, between May and August.