Mizter T wrote:

during the last few months of Routemaster
operation on the 12, some of the actual buses provided were not
Routemasters but 'standard', modern double-deckers albeit with a
conductor (which is what what I meant when I said "OPO double
deckers"!). I don't know why this was the case, unless the bus company
(London Central) had started to give up on doing any heavy servicing
of their Routemaster stock, what with its imminent demise, and so had
substituted other buses.

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.