I've been thinking a bit more about the air cleaner mushroom, I've def seen the same type as mine on other 6600s ,although not as often as the other type, and the ones I saw them on also had power plus stickers like mine so I'm just wondering if the power plus ones had that type mushroom which I think is the same as the 6610 which was just about to be introduced? Maybe somebody on here may know better. Also there are 2 types of air filter for them, my one takes a double filter, a small inner one and a large outer one whereas the other type I think is just a single filter which is quite a bit smaller.
The other thing which makes me think it's original is that nothing was messed with on that tractor, the only thing that wasn't on it that should have been there was the rear work light brackets. The nose cone was completely undamaged so it's unlikely the mushroom was damaged and I doubt it rotted off although when you see the state of the cab you never know.