Cheers guys!
No worries Andy, I knew what you meant (edited my post too - I meant Main spar/ rear wall, not rear spar!)
The subject of P-51 well colours seems to be a minefield ( especially for P-51D Mustangs!)
From research on the net regarding P-51Bs (as far as that is possible with all the conflicting opinions), I agree on the clear lacquered alloy skins and YZC main spar, just not certain on the stringers/ ribs. My impression is these were also excempted from painting when supply of aircraft was critical. I could be wrong.
I'm not sure about the dividing wall between the wells during this time either, which would normally have been a Dull Dark Green in early model Bs/Cs, later Interior Green (ANA611).
The only certain thing for T9+HK is the u/c doors, which definitely look NMF in the photos Paul posted.
As for plumbing, I am *assuming* couplings, etc were originally in their own natural colours, possibly lacquered against corrosion, before the postwar ID standardisation came into being, though certain items were primed in various colours according to net sources. Not sure if these were excempted from painting when production pressure was on or not.
Sounds like much ado about nothing, but it would be a shame trying to make all these things and then go and paint them the wrong colours!!