You're correct, and I thought it was fairly obvious to most that a suffixed route was a variation. I mentioned earlier the 232 and 232A - these shared the same end points (Hounslow Bus Garage and Yeading (White Hart), and followed the same route from Hounslow to the North Star (North Hyde Lane), then split, taking different routes, then rejoining again in King Street in Southall, then sharing the route again to Yeading.
Interestingly, the 90B ran for very many years (probably decades) after the 90 itself ceased running. I think the 90B is just the 90 now, how boring!
Now, thinking back, there was a specific Airport bus route with an 'A' in it - the A1 Express route that ran non-stop from Hounslow West station to Heathrow Central, that was, until the Piccadilly Line opened to Heathrow Airport Central. Used that bus many times for a day out at the Airport (no child fares though!). So in a way 81C is almost correct about 'A' for Airport, but as far as I know for Heathrow, that was the only specific 'A' route, for Airport, to go there.