GK:
All of those C-135s are the same base airframe (the prefix indicates modified mission - a very clear description) - while all of your R1s, GR1s, and B1/B2s are all very different airframes with the same designation.
I know, but that is simply because we name our aircraft rather than just using a designator. Most people refer to the KF-135 as the KC-135 not as a "Stratotanker" (or whatever its proper name is). We refer to our aircraft (usually) by the name so a Reaper is a Reaper not an "MQ-9". Our B2s are not really "B2"s but Canberras, Victors, Vulcans etc. The mark number and designator just indicate role and version. Horses for courses but that is the UK "tradition" and it isn't difficult to understand.
