I was perusing some domestic UK flights, and noticed that the Shuttle flights (appear to) have a callsign of SHTXXY, where Y is a random letter, but XX is a number that corresponds to the route, even being away from LHR and odd towards. For example, Manchester is 2/3, Glasgow is 6/7, and Aberdeen is 18-19. I did some searching, but couldn't find anything - is there any reason for this numbering convention, and why did Manchester get the lowest numbers when Glasgow was the launch city back in the day?