This is the cap and bellows for rear round resv BREMBO master cylinders found on most bevel drive twins. The part number on the underside of the cap is 10.2704.10 and the part number on the rubber bellow is 10.2711.10 - please check yours to make sure they match.
These also fit the smaller front round reservoir clutch & brake master cylinders on Alla'Azurra etc.
Ducati part #s.......
Cap 0820.39.676 (082039676)
Bellows 0820.39.711 (082039711)