Been there done that. I have too many internal dcuments and drawings to refute that clip. We beat it to death, but 539 and 540 fuselage and canopy were identical to P-51B-1 43-12102.
They were P-51B-1 Fuselage Spares with P-51D Wing (Not modified B wing), under Material Change request MCR 258-1, -2 and -3 and sent to Advanced Production/Experimental Department Hanger in July 1943. Each one had the C-258 Cockpit Enclosure, Sliding modiication installed exactly the same. You will find MCR-258-2 Cockpit, Sliding Enclosure in the NA-102 P-51B-1 Specifications and the MCR 258-1 and -3 in the NA-106 P-51D Specs;
The rocket mod you have in that clip is the M-10 - which was not installed on 540 before delivery, only the MX 241 shortly afterwards at CalTech. The Master Drawing sections attached below incorporates the Rocket Stubs for Generic P-51D (Data Block top Right) in 1945 as well as Dorsal Fin.
The anomalies in the clip which I pointed out were: 1.) the P-51B-1 did not have the carb screen/filter on lower cowl, but the video images show them (NA-102 P-51B-1 did not receive mod to retro fit them); 2.) the image does not have the 55gal fuse tank filler cap, 3.) The last time that P-51D-1-NT saw California or any mountains in its travels, was Dec 31 through February 10 at Inglewood prior to the MX-241, (prior to the M-10 stub kits released in July 1944), at Caltech Feb 10 through April 4 when it finished initial prototype (for D) test at NAA and went to Wright Field, then departed for Eglin 4-16 via Orlando where it stayed until January 1945, then Key Field and finally a mid air structural failure June 25, 1945. In oter words 540 never returned to desert/mountains after April 4, 1944. In other words, the first rocket system installed on 540 was the MX-241 and was long gone to Eglin before final resting place in MS.
The P-51B-10-NA #1, 43-7213, received the MX-241 mod in April 1944 after 42-106540 left for Eglin Proving Ground. The first rocket stub/zero launch system was instlled on P-51D-10 and -5 in August 1944 in prep for release as production item on P-51D-20 second block in December 1944. I believe (unconfirmed) that Eglin installed the first zero launch kits on 540 near the end of 1944 for parallel testing to NAA Inglewood, but don't have the paperwork from either Wright Field or NAA to support that speculation
The reason the 55gal filler cap is critical to the narrative is that the AAF granted permission to NAA to install the 55gal tank in August 1943 under NA-106 MCR 326-4, when for some reason there was an interference issue betwen P-51D wing and B-1 fuselage discovered, as modified, for the Sliding Canopy aft deck.
That is the reason BTW that the P-51D didn't go into production under NA-106 with mods from the B-1 fuselage. NA-109 and NA-111 received the P-51D related funds from the NA-106 contract AC 30479, but the majority of drawings for subsequent NA-109 and NA-111 originated in NA-106.
The attached images are from P-51D 106-00001 Three View General Arrangement. Note the Release Date in the Title Block (3-15-43). Note Revision A date Redrawn 11-5-43 (Prior to first flight of P-51D 42-106539 with no fuse tank) and prior to delivery of P-51D-1 42-106540 (Dec 31 1943) with 55 gal fuselage) tank.
Note inthe side elevation of NA-106 P-51D as Redrawn on 11-5-43, that there is neither carb screen or fuselage fuel tank fill cover.