I would like to find a set of lowers but they are tricky too as they are model specific. Th best bet is to find the gear that came off the same bike as yours. Once the original bike is out of the picture there is no way to tell. At least the fairing itself is generic.
The lowers were made from a big blank, with notches cut out for the various models. At worst, you might get a set that has a notch for [nothing] on your bike. 
Thanks HM, I have been wondering about that for a few years.
-Alan
In the mid-1970s, we installed Vetters on CB750, CB500/550, CB350 (both "F" and twins), Suzy GT750, GT550, and the BMWs R75/- and R90/- series bikes. The lowers were all identical where they attached to the fairing, and their lower sections followed the same lines except for those notches: they had BIG notches over the BMW twin cylinders and almost no notches on the GT750 except for the alternator cover. And, they almost fully enclosed the CB350 twin engine, with no notches at all: looked pretty cool back then!
