Since the standardized designation system is gone out the window now
anyway, I won't be surprised if the R is for recon, instead of calling
it an RB-1D(?). ISTR back in the early-mid '80s a B-1C was proposed as a
long range interceptor for Soviet bombers to be equipped with the F-14
radar & Phoenix missiles in the bays & be operated by ADC. That was in a
news blurb in AWST way back then.