If one looks at this page in the original (any other page with music will do as well), we see that sharps and flats in a key signature are in the correct place for a treble clef, so I can only assume that what looks like an oddly placed alto celf is just an old way of writing a treble clef (please anyone correct me if you know otherwise).

I've been playing Branle des chevauux without the f#. After a few plays - to get the modern way out of my head so I stop comparing - it sounds good. I think this comment will make me appear to be a pinhead, but somehow the f natural makes it sound 'more French' - by which I mean in keeping to my ear with other French tunes of the time - though I cannot justify that comment musicologically.