P-51 Being Outfitted For NACA Testing

January 1946. That little model they have there looks a lot like a Bell X-1.

This was a scheme being used at the time for high speed, transonic flow testing as no wind tunnels were capable of trans/supersonic flow speeds at the time.

With the P-51 laminar flow wing, stable air was accelerated to those speeds in that area of the airfoil.

One of those NACA airplanes survived, and has been restored as such...

I knew they had restored one but not that they had kept the test articles on the wing. But you can see them here, too. Note that the exhausts seem to be like an H model.

I'm not sure if it's to illustrate your point about the exhaust shrouding, but the bottom two images are of actual P-51Hs. The fairing of the radiator scoop into the rear fuselage and especially the lack of a kink in the wings leading edges for the landing gear wheels next to the fuselage are giveaways.

Also on the P-51D pictured, it does look like a normal (or maybe slightly modified) standard P-51B/C/D/K exhaust shroud. The fact that the aluminum around and aft the exhaust and the shroud (be it aluminum or steel) is polished does play tricks maybe on one's eyes (it did mine at first, hence editing to add this comment).

A lot of the NACA P-51Ds were fitted with the P-51H taller tail unit (or at least the extension on the vertical stabilizer) as, on the H, it improved handling/directional stability under certain circumstances (an annoying issue on the Mustang dating back to the P-51B due to the switch from the Allison to the Merlin, and use of a 4 bladed prop instead of 3).

Well, on the H models and possibly that restored ex-NACA D model the fairing around the exhaust pipes seems to be bent around the pipes rather than just having the pipes sticking out of it. And I think the material on the H and the restored D is Stainless Steel, not AL.

Remember that the actual P-51H vertical tail was very different from the P-51D tail with cap as seen here.

The first looks like a P-80 shape

I've no idea what shape they were testing with the second one.