The Book Switch Theory

[eggyk]

If the premise is that the ex libris was forged, then the question is necessarily where Voynich picked up that form of the name from. 

Its worth mentioning as well that in the only (known) version of "Jacobi à Tepenecz" does not share an identical "Jacobi" to the VMS. 
There are only a few options that allows a forgery to include both details. 

Option 1: Voynich had access to multiple books/pictures that include at least 2 different Tepenecz signatures, and chose to mix the aspects of each
Option 2: Voynich had access to a seperate, unknown book with an exact match for "Jacobi" and "Tepenecz", along with the knowledge that he also used "à" in his signature elsewhere
Option 3: Voynich had access to a seperate, unknown book with an exact signature match of "Jacobi à Tepenecz"

The most likely of the three, in my opinion, is that he had access to the book with "à Tepenecz" (Dialecta Aristotelis), along with one of the books that seperately matches the "Jacobi" (De Proprietatu Rerum / Opus Ruralium Commodorum).

As @kckluge asks, how plausible would this have been, and how much effort or time would that have taken?