Thank-you! The flap covers a front pocket for easy access to smaller flat objects. It has no magnetic fastener or anything like this at all, so better not to overstuff the front pocket. I think this bag belongs to the same collection as the Mombasa bags. I´ve seen this model with a deer horn handle, too.Oh what a handsome bag! Does the flap drop down? Or is that a front flap pocket? I really like the older YSL...they had that cool girl vibe.
I have another one from the very same time and collection.

Took it to Paris with me last year, but wasn´t happy as the main flap of the bag with a magnetic fastener tends to pop open when I carry the bag crossbody. The doctor´s bag fastener on this year´s model should be easier to use.