We also grow Petraea kohautiana, that many consider a synonym of P. volubilis, but is definitely a different plant - at least the one we grow under that name it is different from the one we call volubilis, both in flowers and leaves, and it is more woodsy and almost self-supporting. Inflorescences are longer, flowers bigger. David have you ever grown it?
I also planted a forma alba - but i cannot tell if it is the volubilis or the kohautiana, as until now it has never bloomed.