John Rowland wrote:
This wasn't unprecedented: for many years in the 1970s/80s two externally
identical buses made up (presumably) the majority of London's bus fleet,
with the OP version designated DMS and the crew-op version designated DM. A
DM would sometimes be used on a normally RM route.

Never the majority, due to their unreliability. They didn't even manage
to get rid of the last of the RM's predecessors (RTs) until the next
generation double-deckers (Ms and Ts) started to join the fleet.

Colin McKenzie

--
No-one has ever proved that cycle helmets make cycling any safer at the
population level, and anyway cycling is about as safe per mile as walking.
Make an informed choice - visit www.cyclehelmets.org.