Thank you Joe - since the time of my initial research into the subject five years ago, I came to realize that what I had been reading about with regard to the "ET" in "ETF-51D" as meaning "Extend Tail" was wrong too, but I never placed where that "E" came from. Of course I could acknowledge that there were tons of stock single seat Mustangs based State-side that were designated as "TF-51D", and that the "E" was the only difference in the designation applied to all of the former NACA Mustangs in post-NACA USAF assignments. Every ETF-51D designated Mustang I have come across in period records are former NACA examples, so I would be interested to see/know of any others that weren't used by the NACA but still got the "E" added.
Here is a photo of 44-13253, the first production P-51D, which was retained by NAA at Inglewood and shows the extended tail fin cap added (also of note is the all-metal rudder and early D/K dorsal fin fillet, added by this later point in time).

Here is a photo of 44-13257, just the fifth production P-51D, shown shortly after acceptance by the NACA at Langley with the extended tail fin cap - which probably/as far as I know had already been installed back at Inglewood with NAA - note the spinner is painted, which was common to Mustangs retained by NAA for test purposes. This aircraft became NACA 108 and later went back to the USAF as ETF-51D 44-13257. It was sold surplus in '57 and a decade later became the Cavalier F-51D Mk.II prototype (and for which the taller tail fin cap design was copied/reproduced on other Cavalier-modified Mustangs). It of course remains flying today with the Lindsay family, registered N51DL.

Here are a couple photos of Bill Allmon's P-51D NACA 127 shortly after arriving at Langley with the NACA and prior to having the extended tail fin cap added.


Here is another photo (to go with those posted by Mark) of 44-14017 NACA 102, the one and only P-51D that had previously been borrowed by the Navy in the fall of '44 for carrier trials. In this photo you can still see a few hints of the modifications/additions left-over from the tail hook installation, such as the unique cutout in the base of the rudder, which had been done in '44 to make room for the catapult holdback bar mounted at the base of the tail. This aircraft appears to have been written off after a landing accident in December 1953, when operating with a USAF unit in New York. (Note the Bell X-1 style cross section model mounted to the wing for testing.)
