Sarfatti and Ponti for Stilnovo?  

Are you looking for a specific lamp by Stilnovo? Posting a picture might help.

Gino Sarfatti is associated with Arteluce a company he founded(?) which later became part of Flos. Gio Ponti designed lamps for Fontana Arte & one for Artemide. If there ever was a collaboration between the two, it would have been public knowledge by now. I have heard of Gino Sarfatti working with other Italian designers such as Franco Albini, Ico Paris, Sergio Asti, Gianfranco Frattini. Gio Ponti's most well known collaboration is with Piero Fornasetti. All three lighting manufacturers (Arteluce, Fontana Arte, Stilnovo) from that period may appear at first glance produced similar light fixtures but a closer look will reveal that each have a very distinct/unique Italian style.

You might want to check Taschen books, 1000 Lights vol.1, you might get lucky that the lamp you are inquiring might be included. Or the light fixture section of other Tascen books, 50's, 60's, 70's Design.