18 ct gold nib model - for the French market

In the French market, only nibs made of 18 carat gold or more were referred as "gold nib" (Pelikan-collectibles)⁽¹⁾. Not an exception, Pelikan fountain pens were equipped with 18 ct gold nib for the French market. Here are some models with 18 ct gold nib (model 100, 100N, and 400). Some nibs have imprints "Angleterre" or "England"⁽²⁾. Interestingly, as far as I confirmed, all nibs lack millesimal fineness (e.g. 750), different from 14 carat nib of the same time. Some pens have "Pelican" imprint on the cap top in line with 18 carat gold nibs (see "Pelican" model).

Note;⁽¹⁾Many sources mention it, but I was not able to track down its primary (original) sources. Why 18 carat gold or more? Was that regulated by law, decree, or commercial practice? How long was it in effect? Why the French market?
⁽²⁾They were made or engraved in England?