Two unpublished letters from 1460, sent from Siena by Bartolomeo Bonatti to the Gonzaga family of Mantua, whom he served, describe the Easter celebrations conducted by Pope Pius II. These archival sources provide details on the customs of the papal court present in the Tuscan city at the time. Among the vestments displayed on the occasion, one is described as a very unusual fabric, painted by Fra Filippo Lippi. This paper analyses the letters, their content and context, and proposes several hypotheses regarding this previously unknown work by Lippi, commissioned by Pope Nicholas V. The vestment can be traced in a few later 15th-century inventories, but it is now considered lost.