Rupert Candy wrote to uk.transport.london on Mon, 4 Apr 2005:

This is the case with quite a few bendy routes now - the buses for the
73 and 25 were delivered with card readers fitted at all doors, and
they were commissioned a few weeks ago, and the 521's Citaros seem to
be having them retro-fitted. IMO an excellent idea and one that should
have been put in place before the buses were put in service.

As they have been in every other city in the world with
multi-entrance-door buses, whether bendy or rigid!
--
"Mrs Redboots"
http://www.amsmyth.demon.co.uk/
Website updated 3 April 2005