Morten St George Theory

[Koen G]

However, some culture creeps in, and it helps give us clues.

This is a crucial part to understand. I am absolutely convinced that if you were to present Virgo, Gemini and Sagittarius to a curator with the relevant expertise, they would date it to 1400-1430, give or take a decade. 

It's still possible to say the MS was made later. But this would already require two violations of Occam's razor. You'd have to add the condition that old vellum was used, and that old fashion was used, which happens to overlap with the period of the vellum in the first place.