(16 Aug 2019, 1:21 am)Washingtonian wrote
I might be wrong here but I think the A in 49A is for the Anti-Clockwise part of the route, while the 49C is for the Clockwise part of the route. That's why I think they use A and C, instead of A and B. I'm sure this was the case for the old Silver Arrows Sunderland - Washington services 2A and 2C. This numbering used to annoy me too until someone explained it.
This system was also used several years ago in North Shields when local circular services 83A and 83C existed. Makes sense.