The origin of Fabrizio Salani's "AG" monagram

[Fabrizio Salani]

Therefore, it seems likely that the design of the seal originated after Renoir's publication, but independently from its adoption by the UK company.

It is very likely that the exact seal we're looking for is featured in one of these Dover publications. Since the UK company referred me to the 2004 edition, this or the 2012 edition may be worth tracking down.

For whatever it's worth, the seal for "AD" from renoir features the rounded loop seen in the UK "AG" version. Maybe at some point they expanded or blended the designs. 

I found the 1970 edition, but it features exactly the 1870 edition, plus other authors with different monograms. Now we must ask ourselves where Henri Renoir drew inspiration for the monograms: whether it was a collection, a fantasy, or modifications of existing monograms. I haven't been able to find any information on this, especially since it's always cited as a collection.