3.5 Planars and Xenotars are Bayonet II (2), often called 'Bay II.' T Rolleiflexes are Bayonet I (one), Bay I, as are other Tessar and Xenar models. 2.8 lenses, Planars and Xenotars, are Bayonet III. The first 2.8 Rolleiflex, a 2.8 Tessar, was a Bay II but the flange orientation (and spacing?) is different than on later Bay II models. Bay IV was used on the Rolleiwide.

There is also an issue with spacing between certain E Rolleiflexes and F models. I don't know the particualrs, though, but be warned. The spacing for hinged lens caps changed at some point.

Rolleinars are labeled with two sets of info. One is the strength- 1,2,3. The other is the Bayonet size- 1, 2, 3. Much clearer, yes?? Well, my Bay II Rollienar 1 is actually labeled 'Rolleinar 1' (that's the numeral one) and then a fancy cursive 'R' followed by two sans serif capital I's.