Oh, Antonius, the photo and so accurate description of your Pan Bagnat provoked such an outburst of nostalgia that they almost made me cry... Even the round bread looked perfect .
The only thing that did not fit my memory of that sandwich (I hate hundreds of them in my student years at the University of Aix-en-Provence) was the basil. You would not find a single leaf of basil in those I bought in street kiosks or in bakeries. But in the bakeries of Nice you would.
Anyway, I would like to mention a few details about the origins and the name of that sandwich that in "Nissard" the old Occitan language of the Nice area, means ''pain mouillé'' that you can translate by '' soaked (or wet) bread''. In some Italian regions you find an equivalent with the '' Pane bagnato''. The wetness came of course from the olive oil that was liberally doused on the bread, but also from a little red wine vinegar.
In fact that sandwich whose recipe you will never find in any ''bible'' of Provençal cuisine such as the Reboul, was a
'' casse-croûte'' that poor fishermen and daily laborers took with them in the morning on their way to work.
Originally, from the 18th century until the 1930's, the fish component was, as you rightly indicated, filets d'anchois (anchovies). But after WW II the use of canned tuna, that was no longer an expensive fish, progressively replaced the anchovy.
Nowadays, with the depressing state of what fast food for the tourist trade, even in real ''sandwicheries'', has become, when you buy Pan Bagnat from street vendors and supermarket bakery departments in Paris or even Marseille, you most likely will be disapointed by both the quality of the ingredients which can be anything on hand in the fridge and not necessarily fresh vegetable or even olive oil, and the bread that will be a cheap chewy industrial kind.
I believe that in the old district of Nice you still find the right think.