In Andalussia it´s different, there you can find sometimes individuals with ocells but often they have dark spots where they should have the ocells. In Morocco too and in central and north Spain. Maybe exist a valley or something like that where you "always" find individuals with ocells or striped individuals (bilineata, 2striped design). Maybe in Sardinia, where there aren´t big genetic movement, all the individuals have ocells. 4/4 in your trip is very casuality. And the individual of Wouter and Philip de Pous of Tunissia had ocells too, so this must be typical characteristic of the Tunissian clade.
Anyway they are "Natrix maura", what are we doing talking about that???
Cheers
Gabri