I haven't seen that before, very interesting. Good questions, I haven't ever heard why they would do that? I don't know how much actual gunnery practice they got back then, it wasn't like it is today though. Maybe this unit was for learning combat maneuvers and tactics?
Before WWII vintage photos on this page are the only ones I have seen, and the one with the gun covers in the black and white, I didn't see until this unit's main page.
It's an easy modification to do, it would be unique from other P-40's, and you have photographic proof of it, so it was at least used on some aircraft. I would go for it