Just a quick visual idea for you to ponder on with regard to Voynich Manuscript page f57v: it’s something I’ve not seen mentioned elsewhere.
Back in 2010, I posted a page here discussing astrolabes, nocturnals and Voynich Manuscript page f57v, in which I laid out some codicological reasoning why I thought the 4 x 17 = 68 single character ring was actually a 4 x 18 = 72 mark ring, i.e. marks spaced every (360 / 72) = 5 degrees. (I also didn’t explain nocturlabes as well as I should have done, so that’s something I ought to return to soon.)
One other anomalous feature of f57v is the text in the innermost ring, three quarters of which is also made up of single characters (marked in red below). This looks to me as though as though it too might be concealing a string of marks. But on what kind of device would marks only go three quarters of the way around?
So… your Voynich thought for the day is that there is indeed a very specific type of device of great interest in the fifteenth century where the marks only go 75% of the way around: a sundial (or solar clock), which very often only cover 18 hours of a day.
Now, I’m really not saying that f57v is ‘definitely’ a sundial (in the world of the Voynich Manuscript, nothing is ever that easy): but, rather, that the idea that at least one of the text rings on this page might well be somehow connected with a sundial ought (I think) to be considered here.
I don’t recall any other theory or suggestion that explains the curious string of characters on the innermost ring: nor why (for example) it should contain freestanding EVA ‘l’ shapes, even though these hardly ever appear elsewhere in the text, or various other unknown weird characters. My strong suspicion is therefore that these are just random letters added to cover up dots and dashes in the original diagram, and have no actual meaning beyond that.