I don't know if it was universal, but when I went to secondary school in about 1965 it was nearly an hour's journey each way on one or more London Country (Country Buses?) RTs.
The bell signals were 1 ring from anybody for a request stop, 2 for "right away" (obviously from the conductor only) and 3 from the conductor/conductress: "Full up: ignore people at request stops!" I was very glad that I boarded before it became F&S.
Now this is a bit niche, but around the same time my school journey involved one of the Aldershot & District's 34, 34A, 34B or 48A services which at that time had a common section of route between Woking and West End. They were mainly operated by Dennis Loline III vehicles, which were half-cabs with a front entrance and a sliding door operated by the driver. There was a small window between the driving cab and the platform where passengers stood waiting to alight (at the front of the downstairs saloon).
During weekdays and Saturdays, these routes were almost wholly the preserve of Woking depot. Conductors would use a coin from their change bag to tap on the small window between the platform and the driving cab, one for stop and two for go; the bell was never used. In the absence of a single tap approaching stops where no-one was waiting, the driver would glance over their left shoulder to see if anyone was waiting to alight. And self-despatch from stops if necessary.
On Sundays much of the work on these routes was allocated to Guildford depot and it was only when I travelled on Sundays that I realised that the practice appeared to be unique to Woking depot. Of course, there may be forum members who know of a similar practice at Aldershot, Alton, Hindhead etc.
I did say it was a bit niche...