Illyrian Illyricum

[ReneZ]

But how do we know this is about the VM in the first place?

This is how we know:

six weeks after Kircher wrote and sent this letter to Moretus, Barschius wrote to Kircher saying that he heard from Moretus that his earlier letter had arrived. He then goes on to describe the Voynich MS.

Only if one believes that the Voynich MS is a modern fake by Voynich...

All details, including image of the Kircher letter taken in 2016, new transcription and new translation, and the full context, are explained here:  You are not allowed to view links. Register or Login to view.