Gah, you're right I had forgotten it was recessive. Sorry.
My understanding is the Isabel pattern is Partridge based with the Lavender gene. Recognized in Europe in Brahmas and...Leghorns?
If that is the case, theoretically you could breed partridge into them, and test mate to see who is carrying a hidden single copy of vulture hocks. Cull like a mad man, and be very carefull not to get that fault started in partridge as well.
Type wise are the partridge over here coming around much ? Being a variation of darks probably not helping them as much there, as if they were crossing to lights, or buffs even. Can't tell from the single photo of adult bird in the add if these Isabel's have any type either as far as that goes.