Hi Kari,
"Fond" is a French word meaning in that use "background" of the dial.
So basically "fond blanc" means white background dial.
"Porcelaine" is the name of a material, translated in English by "china", used for making dishes.
I guess it means that it is a white dial looking like the aspect of a white china dish surface.
In my older PP catalogs the 3520D is not indicated with a "Fond blanc porcelaine" or "cadran blanc porcelaine". References 3919 or 3802 had a "Cadran blanc porcelaine" in the catalog.
3520D is written to have a "Cadran blanc laque" meaning a white lacquered dial.
I hope it's clear.
Dje