Well, that's one version of events. OK, it was going to be RS at one point in the process.
I personally have my doubts about the RS legend. If it ever was RS-71 then the "S" would be the basic mission designator and "R" the modifier. "S" has always been (since 1962) the basic mission designator for anti-submarine warfare aircraft, which makes no sense when applied to the Blackbird. "R" has always been the basic mission designator for Reconnaissance. The confusing part is that "S" is the modifier for anti-submarine warfare, which also would not seem to properly apply to SR-71. But to me it makes more sense to have the modifier ("S") deviate from the standard and stand for "Strategic" rather than "anti-Submarine", than to have
both the basic mission designator and the modifier deviate from the standard. But who knows for certain? That's what's fun about these legends.