Originally Posted by
Hartington
My memory suggests that the original format of the Shuttle call sign was along the lines shuttle 1 Alpha where 1 indicated the route and direction (so, for the sake of argument 1 = LHR/GLA and 2=GLA/LHR, 3=LHR/EDI etc). The letter indicated the sequence so 1A was the first LHR/GLA 1B was the second LHR/GLA.
That's pretty well how it still works.