About '13' - of course there are 13 4-week months in the year, but whether that's to the point, or compatible with system of calculation used at the time and place the Vms diagram was first created is something you'd have to look into. 
Seems to me that you've imposed on the drawing a type of diagram that's common enough in medieval texts about science, but which has never yet assisted our knowledge of the ms (though it has helped pad out holes in a number of tenuous theories).
i recall a lot of dust and excitement about one diagram of the sort - didn't explain anything about the written or the pictorial text in Beinecke 408, of course, but it was fun. Here's that diagram.
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Seems to me that you've imposed on the drawing a type of diagram that's common enough in medieval texts about science, but which has never yet assisted our knowledge of the ms (though it has helped pad out holes in a number of tenuous theories).
i recall a lot of dust and excitement about one diagram of the sort - didn't explain anything about the written or the pictorial text in Beinecke 408, of course, but it was fun. Here's that diagram.
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