Having driven both the 77 and 77A, I cannot recall any passengers complaining that they had boarded the wrong bus. Which would seem to indicate that there was not in fact any issue.
On the other hand, my partner is dyslexic, and was repeatedly confused at Turnpike Lane by the existence of a 123 and a 231. I would imagine far more people, of similar affliction, would find that and other similar examples more confusing than 77/77A.
Exactly, and TfL's non-acceptance of 3 or 4 track route number blinds made the decision even more nonsensical, so that an 'A' could not be accidentally included or excluded on a blind.
My own candidate for 'unique' London bus route status is the 76, partly based on its routeing over many decades although recognizably the same route now as then. My reasons are as follows:-
Along with a handful of others like the 16,24 and 38 it has plied the same streets for more than a century and, unlike the 24, it is operated by the same garage now as when it started i.e. Tottenham (AR) and ,apart from a brief hiatus after privatisation, has always been worked by that garage (rather like its number-neighbour 75 at Catford).
It was the only London bus route which you could see crossing two adjoining bridges (Westminster and Blackfriars) simply by standing in the middle of the bridge in the middle, namely Waterloo. This was when it ran from Victoria to Tottenham via Waterloo and Queen Victoria Street. To anyone who might ask 'how could you tell which route the buses were on?' the answer was simple when the 76 became the first London route to be operated by rear-engined, doored doubledeckers the XA (Atlantean) class, experimentally. They were later joined by the truly-unique FRM1.Nowadays. of course Westminster (also Lambeth, used on diversion during State occasions) is long gone as, more recently (and needlessly imo) is Blackfriars, the route to Waterloo now joining the common herd over the bridge of that name.
Other reasons why it stood out from the crowd: the 76 was jointly scheduled with the works-journeys 34B route, so buses worked onto and off that route rather like the better-known 15 and 100 at Upton Park. It also had a separate peaks only schedule from Victoria to Liverpool Street Station to give Blackfriars Station passengers a direct route to the latter and, unlike the 25 journeys from Dagenham East Station to Fords which were always worked off the main schedule, I believe the 76s were worked as a supplementary schedule i.e. with its own allocation, rather like the NBfLs on the 38 were at first.
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Another 'unique' route is the 521, the only one to use part of an ex-tram subway!