Lovely land mine you stepped in Steph. Until Wojtek steps in, I see the small vent on the first production Yak-9 up to the Yak-9D. The bumps(?) in front of the windscreen are also there on these variants and disappear on the later variants as well. I don't see them on the Yak-7B drawings in the AJ Press book, "Jak-7 Jak-9". I'll keep looking.
Geo
EDIT: I found the same photo as your top photo in the above mentioned book and it says " somehow this image is awful, but the Yak-9d is an interesting modification - the rear view mirror in cabin"