Is the 750 frame identical to the 906 frame? And if so, where is the proof of that?

They aren't identical even if very similar.
It's enough thinking about the front side, where the 906 has a central water radiator while the 750 has two oil radiators on the side.
Ciao

It got me thinking. I looked in the part catalogue and could see a miner deferens, as you identified Francesco the cooling system on the PASO’s are different which can be seen as two small brackets on the front of the frame, which also may explain the different part numbers. Other than that they look very similar.
Ciao