The LBJ gaff is urban legend. Col. Rich Graham, an SR-71 pilot and commander of the 9th Strategic Reconnaissance Wing, did some research on this very matter. He obtained the original text of LBJs speech and tape with everything correct as SR-71. The official transcript released to the press differed with "RS-71" in place of SR-71. Apparently, the stenographer had heard incorrectly and recorded "RS-71" in all three places.
Oh yeah?
Immediately prior there was an RS-70. Never were anything in a series before the SR-71