Interesting shots! There was a great article on the restoration of the XP-51 in the EAA magazine several years ago/ I'll have to dig it out.
The testing at Farnborogh showed that the airplane was too hot! As in temperature. There was nothing separating the cockpit from the radiator down below. Hot air tended to get sucked up from the radiator past the pilot and out the rear cockpit vents. I suspect this may be why the Merlin powered versions had a plywood floor, while the Allison versions had the seat bolted directly to the wing with no floor. The P-38 was too cold to fly and the Mustang was too hot! I would guess that they may have put a fabric bulkhead below the cockpit on the earlier versions in order to keep the heat out.