IIRC, it bolts onto the frame, near the sidestand bracket, #29 in the drawing below, switch up, the plunger is depressed by #26 - the lock at the top of the sidestand's shaft.
Should be the same as on Converts and G5s, so maybe someone here can shoot a photo? If they still have it on the bike. If not, give me a day or two and I'll go over to my friend Leo's and shoot some photos of his.
IIRC, it wires into the circuit going from the right handlebar kill switch and the coils. This might help (even though it's for a Convert). The sidestand switch is #53 at the top of the diagram.
http://www.thisoldtractor.com/guzzi007/pdfs/1975_Convert_USA.pdf