||Sei quintetti per tre violini, violoncello obligatto e' basso repieno, opera seconda
RISM interprets basso as violoncello (not unusually) and reads this as 3 violins and 2 cellos (the first cello a concertato - solo - cello and the second a ripieno cello - one that takes a more accompanying role- think concertante and ripieno groups in a concerto grosso. Also, the E♭ quintet ms copy has: Sonata da Camera à 3. Violini, e due Violoncelli Obligati which tends to support this view...) - Schissel
However, as the Basso part has figured bass, it could conceivably be played by other continuo instruments.