Richard J. wrote:

(snip)

I thought that the compulsory/request distinction referred only to
people waiting at the stop, and that passengers on the bus were expected
to ring the bell for any stop.

I demur. I was always under the impression that a compulsory stop meant
that technically speaking the bus should always stop, so theoretically
there's no need for passengers on board to ring the bell.

Also drivers often seem/seemed to adopt the practice of slowing down
for a compulsory stop, and if no passenger on board looks/looked like
they are/were going to get off (i.e. no one near the door or standing
up) and there is/was no-one at the bus stop then the driver will/would
just continue without stopping (the choices in tense for that past
paragraph reserves judgement to the reader as to whether such a
practice continues today! - I'd say the practice still happens some of
the time).