I believe that Monviso was the brand of Pierino Sandri. Unfortunately I know next to nothing about him. He patented a rod type derailleur design in the late 1940s and went on to manufacture a development of it. I have seen photos of several examples of this derailleur - so it may have been produced in some volume.

Monviso, or Monte Viso, is a towering mountain in the Cottian Alps, south west of Turin - so these derailleurs follow in the footsteps of many others, in being named after titanic mountains and their terrifying climbs.