Old monogram on the book "JVS SV".
The portrait is probably of the workshop of Justus Sustermans, a Flemish painter from the Medici court in Florence, best known as a portrait painter. The most famous portrait of Cardinal Carlo De Medici is kept at the Poldi Pezzoli Museum, of which many repetitions are known.

Reference bibliography: Marco Chiarini, Claudio Pizzorusso, Sustermans sessant'anni alla corte dei medici, Centro Di, Firenze, Palazzo Pitti, Sala delle Nicchie, July-October 1983, p. 22.