Re London lettering....racking my brains. All sorts of inconsistencies
And there was a W21 in Walthamstow (though Waltham Forest would do for this too)... And some of the "Wood Green" Ws don't go close to there; some never have...
And C has had various uses... originally, in Croydon; then later "Central", but the C2 was the "Camden hoppa" when it (most foolishly) was converted to midibus operation; not sure what the C in C11 was intended to stand for - Camden (borough) again, maybe; I might have suggested Cricklewood but I don't think it went there when it started...
There were, briefly, B routes in Barking. And for much longer, a B1 in Bromley.
Technically, the H1/2/3 are Hampstead Garden Suburb, quite a different place from Hampstead itself...
There was an M1 in Merton/Mitcham/Morden
The P4 didn't (and doesn't) go near to Peckham....
There are some R's in Richmond
There used to be not just an S2, but also an S1 and S3 in Stratford. Think there was another unspecified "S" somewhere round Lewisham too relatively recently.
77C was the last route to have a C suffix, I think on two separate occasions - (in its most recent incarnation it was a schools service) - odd that 77A was the last suffixed route overall, too.
One peculiarity, which didn't last long, and which think was the only example of this is London, was the 228A and 228C - a circular route, with "A" Anticlockwise and "C" Clockwise
The first use of lettered routes, in 1968 (Wood Green) was to indicate one-man operated, flat fare buses; the first minibus routes, introduced a few years later, were also all lettered.