From Snopes research, they seem to go along with it.
........As to why it's called SR-71 and not SR-71, here are some quotes from Lockheed SR-71: The Secret Missions Exposed, courtesy of
Mary Shafer on AFU (the material in square brackets are her comments)
quote:
During the initial stages of assembling the YF-12 in late 1960, it became apparent to ADP [Advanced Development Projects, better known as the Skunk Works] engineers that the basic interceptor airframe could be adapted to provide a strike bomber. Russ Daniel approached Kelly [Johnson, head of ADP] with the idea and asked to write a basic feasibility report.
Kelly reviewed Daniel's B-12 proposal with Strategic Air Command's Commander-in-Chief (CINCSAC) General Curtis LeMay, who agreed to fund R & D [Research & Development] studies provided that these projects would not be used to harm support for the XB-70 Valkyrie bomber program [which is stunningly unstealthy, with huge wingtips that deflected downward at right angles, two big verticals also at right angles, non-moving canards at right angles, and a delta wing with only small dihedral].
{ snip }
Sometime in 1966, the B-12 was christened the RS-71 (RS for Reconnaissance-Strike and the number '71' indicating a follow-on from the RS-70 Valkyrie, which was formerly the B-70). The lack of weapons procurement alarmed Lockheed, who produced drawings of a pure reconnaissance variant, designated the R-12.
{ snip }
[More snippage, mostly of discussion of the political situation during the election year and the transcript of the White House press release inwhich the airplane was called the SR-71 and described as providing a long-range advanced strategic reconnaissance (i.e. SR, as it wasn't Johnson misspeaking) plane for military use.]
Although the political wrangling continues, the future of the R-12 was being solved by Goldwater's taunt. Johnson had conveniently (and politically) transposed 'Reconnaissance-Strike' into 'Strategic Reconnaissance'--hence 'SR-71', which was really Lockheed's R-12. Unfortunately, the B-12 was lost to the McNamara era.
So, from this source it looks like the switch from "RS-71" to "SR-71" was
deliberate, not an accident at all.
1962 United States Tri-Service aircraft designation system
Non-systematic or aberrant designations
SR-71
The SR-71 designator is a continuation of the pre-1962 bomber series, which ended with the XB-70 Valkyrie. During the later period of its testing, the B-70 was proposed for the reconnaissance/strike role, with an RS-70 designation. The USAF decided instead to pursue an RS-71 version of the Lockheed A-12. Then-USAF Chief of Staff Curtis LeMay preferred the SR (Strategic Reconnaissance) designation and wanted the reconnaissance aircraft to be named SR-71. Before the Blackbird was to be announced by President Johnson on 29 February 1964, LeMay lobbied to modify Johnson's speech to read SR-71 instead of RS-71. The media transcript given to the press at the time still had the earlier RS-71 designation in places, creating the myth that the president had misread the aircraft's designation.
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