I think you're right on that.
I think the Fleetline had a designation that would be FE30AGR, indicating that it was an automatic, but Nationals were designated on the design upgrade as you say. Outwardly, aside from some differences, there was little obvious to highlight an original vs an A series. The B series was markedly different but few firms took them, most notably London Country and Crosville.
When you look at it, the B series was only in the 10.3m length - which was the less popular length (even for the A series). London Country liked that because it had always struggled with 'long' single deckers - a hangover of its LT roots in some ways and the 10.3m National was much closer in length to its beloved RFs than the 11.3m.
Other companies had been operating vehicles like the Bristol RELL which meant the move to the 11.3m National was much easier as they'd been used to circa 11.0m buses already.
The B series did attract a few followers though - in addition to the two you cited, Ribble, East Yorkshire, West Yorkshire and Bristol all took numbers of them - probably where in the past they'd have taken Bristol LHs but the B series was more standardised, had better passenger access and might even have been a bit nicer from a driver's perspective.
That whole 'short bus' single deck mentality in the old London Country area hung on for ages - it was only as the Nationals disappeared and things like Lynxes and B10Bs turned up did it change. Interestingly some of the LCBS successors bought second hand Nationals - a number of which were 'long' versions - a type LCBS had shied away from.