The origin of Fabrizio Salani's "AG" monagram

[eggyk]

From all this truly remarkable and thorough research, I believe I can draw one conclusion: it is not an Italian monogram, nor one used in Italy or by the Catholic Church of Rome, but rather a French or English one, or at least it all points to Northern Europe rather than Southern Europe; but perhaps this is just my guess.

Indeed. I found the oldest digitised original of Renoir's work too by the way, as opposed to the english reprint. It is from 1865. 

It's digitised on Gallica as "Collection complète De Chiffres et Monogrammes, par H. Renoir élève de S. Daniel graveur": You are not allowed to view links. Register or Login to view.

Renoir also released "Chiffres Louis XIV" You are not allowed to view links. Register or Login to view. . Archive has it listed as 1865, but another book states that it was actually 1872: 

A machine translation from p92 of this book: You are not allowed to view links. Register or Login to view.

RENOIR (Henri-Pierre), engraver, designer, heraldist, and monogrammist, was born in Limoges on February 11, 1832. Pupil, son-in-law, and successor of Mr. Samuel Daniel, Renoir is one of the best engravers of our era in the industrial arts engravings in the Louis XIV style and in the industrial arts.
-
This artist published in 1865 a collection of ciphers-monograms. In 1872, another album of ciphers in the Louis XIV style. And finally, in 1875, an album of flourished ciphers.

To my eye, the Louis XIV monograms are more intricate with more flourishes, but aren't drastically different in style, which may be because the same man drew them. But the style of the AG seal is very likely french, in any case.

Renoir does sign himself as "student, son-in-law and successor of Samuel Daniel", who is effectively a ghost online with almost no mention of him. But to be the father-in-law of someone born in 1832, he surely would have been born in either the late 18th or early 19th century.