This is a SIEM Optic Unt type 4677 for the MV Agusta 750 S, Moto Guzzi V7 Sport 850 T Lemans, Benelli 650 Tornado, Laverda 750 GT S SF and believed 750 1000 SFC models of the early seventies. Window is 170 mm. Rare model that is not easy to find.  To restore, see pictures. 

Ducati roundcase, Imola, Greenframe, SS, SD, GTS, MHR, SSD, Darmah, squarecase, S2, sport, supersport, Borrani, Ceriani, Scarab, Lockheed, Marzocchi, Brembo, monza , diana, mach, mach1, dm250, dm350, dm160, 160, 450, dm450, desmo, desmodromica, elite, elite 200, brembo, 900SD, Mark 3 , mark 2, CEV, 750, 750GT, 860GT, 900GT, MV Agusta 750 America GT SS