The origin of Fabrizio Salani's "AG" monagram

[Fabrizio Salani]

My feeling is that the seal postdates Renoir, in which case you are correct that we'd be looking at a 20th/21st century copy. 

But there are still ways out of it: maybe someone decided to take an ancient parchment with an ancient Voynich copy on it, fold it in half to use it as a binder for other documents and put their seal on it. (Although the analysis of the pigments might rule out this scenario - I've heard conflicting takes on this).

One could also hypothesize that the "Renoir+" design predates Renoir, but I haven't seen any good evidence that this should be the case. At least not by any amount of time that would significantly change the narrative

I am not an expert in sphragistics but it seems to me, at first glance, that the seal on the parchment is much simpler and less elaborate than Renoir's more graceful versions, and I would see it more at the basis of Renoir's studies than at the end, at least that's my impression.