That would be SR-71, not SR-72.
(Actually was RS-71, but someone (LBJ?) mixed up the letters and "SR" stuck)
Yes, the Recon and Surveillance (RS) 71 aircraft was a technological marvel at the time and LBJ slipped up and transposed the letters. This was years before the "gotcha" journalism era and a time when deference and respect for the office of the President caused the Lockheed officials to scramble to change the designation from RS-71 to SR-71, even changing the painted letters and logo.