It was the RS-71 to follow the then code. But when the President revealed the program he called it the SR-71 by mistake.
So they renamed the aircraft.......
Well, that's one version of events. OK, it
was going to be RS at one point in the process. LeMay preferred SR, and to cut a longer story short the documentation was changed to reflect this. The press release still had 'RS' in places leading to the story that Johnson had himself transposed the letters in the speech he gave.