I am not convinced by the provenance history of the VMS

[Jorge_Stolfi]

Also, Kepler was in the Court of Rudolf, and in his 611 "Dioptrice" actually illustrated the function of a two lens microscope!:

I stand corrected; according to Wikipedia, two-lens microscopes were developed only a couple or decades after the invention of the two-lens telescope, in ~1608.  So it is theoretically possible that a forger in the mid-1600s created a manuscript that pretended to be the work of some forgotten genius who invented the instrument in the 1400s.

(But in that 1611(?) Kepler book, is he describing a microscope, or a telescope?  Unfortunately I cannot read Latin.  From the date I would guess that it was still a telescope...)

we have common ground. On the probable fake nature of the "signature", along with the culprit who put it there. ... So to the "signature", I'll add a possibly supporting, although entirely speculative concept, involving the situation with the "chemical treatment", and the faint nature of the signature to begin with: ...

As a "trailer" for my promised post on that topic:  What made me switch from "just possible" to "highly likely" was a comparison of that signature with that on the "#4" book.  On one hand, the two are just too similar in overall shape, layout, style, etc.  On the other hand, the letters on the VMS are more crooked and uneven, as one would expect in a signature forged by an amateur. 

So here is what I now think happened.  Wilfrid's problem was that there was nothing in the VMS that indicated that it was Baresch's book, much less that it was the "Bacon Original" that Rudolf had allegedly bought for the 600 pazuzas.  No library stamp, no marginal note, no catalog entry.  And no explanation for how a Bacon Original could (1) have got from England to Prague and (2) passed from the Royal library to a nobody like Baresch.  

For (1), he found a barely plausible solution in John Dee, a known Bacon collector who had visited the Emperor in Prague at what looked like to be about the right time. (However Dee returned to England in 1589, which is 10 years before Rudolf's big book purchase identified by Rene, which is probably what Raphael had heard of.)  

For (2), he presumably canvassed the members of Rudolf's court looking for someone who could have taken the book out and whose signature he could forge.  My theory is that he picked Sinapius, because of his role in the Court, and/or because he managed to get what he thought was the doctor's signature from the #4 book that was easily available from the Czech National Library.  He then copied that "signature", changing only the number to #19, as he extrapolated from the likely dates.

But since then we obtained two other books with Jacobus's name on the front page -- from the Strahov library (which would have been much harder to get to).  And one more from the library at Charles University.  And the problem is that the two Strahov "signatures" are in a different hand and language, and none of the first three matches Jacobus's actual legal signature, which is on the fourth one.  And the numbers in those "signatures" are between 4 and 19.

So maybe Wilfrid, after forging the signature, saw copies of some of the other three, and realized that he had copied the wrong signature, with a number that would be hard to explain.  Or maybe he realized that his amateurish forgery would be easily spotted by any handwriting expert, even in 1920.  So he erased the signature, with the excuse of trying to enhance it.  He left only enough of a trace to be barely legible -- but not enough for forensic analysis.

All the best, --stolfi