NASA operated two YF-12As, 60-6935 and 60-6936. 06936 crashed on 24 June 1971 and was then replaced by an SR-71, 61-7951. Because NASA was not allowed to operate the SR-71, it was given the fake designation YF-12C and spurious tail number 06937. The real 60-6937 was an A-12 (c/n 131) of the CIA that was no longer active. 61-7951 remained with NASA until 1978. The second aircraft on the photo is obviously 60-6935. It carries a cooled cylinder which was being exposed to the friction and heat of Mach 3 flight. Photo from: NASA Ames

Constructed in October 1964, first flight in March 1965. Entered servive with the USAF in 1965 as 61-7951. Loaned to NASA around May 1991 as an YF-12C with false serial 60-6937. Back to the USAF in October 1978 and redsignated SR-71A. Loaned to Pima somewhere between 1978 and 1991.

This is the second SR-71 to be built and is now on display at the super Pima Air & Space Museum.