Koen, I think if you look for wide sleeves with tight wrists, you will find comparative examples anywhere that the weather or climate is cold.
The style was first adopted in Europe during the 'mini-ice age' as people needed to wear more bulky underclothing and much more fur. The baggy sleeve holds more warmth; the tight wrist stops it escaping.
This means that the style itself is not unique to western costume, though compatible with early fifteenth century custom and doubtless acceptable - more or less - to the persons who copied the material now in Beinecke MS 408.
Just at random I looked for hunting costume in the colder north and turned up this (modern) version of the traditional Tibetans'.
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It's just an illustration of the scientific principle that comparable problems produce comparable solutions. I'm not arguing that the MS was made outside Europe - though it could have been - but only that you cannot pin-point a date of first composition by this means. Consider all the points at which these figures differ from the usual sort of western Christian drawings of people. The proportion of head to neck to torso for example, or the combination of garments set on a single figure.
For what it's worth, I think the month-centre figures in our present ms were copied from fourteenth-century precedents, with 'updating' done only to remove or fix details which would strike a fifteenth-century westerner as odd or objectionable. But that's just my conclusion from my own work, and you are welcome to ignore it.