In message , Paul Corfield
writes

Sorry, sent all that TWM stuff before I saw that others had filled in
most of the details.

However, to add to this.......
The 11 in Birmingham goes up to 11E doesn't it? So the Outer Circle can
be divided into sectors?

As someone else has said "E" indicates a short, extra?,
"Exception".

trip on TWM routes. TWM don't seem to use letters for local networks
or anything else such as X for express - they are simply buses in the
9xx series.
That's the sole survivor of a system dating back to early WMPTE days:

1-199 South Division (Birmingham & Solihull)
200-299 North Division Dudley
300-399 North Division - Walsall
400-499 North Division - Sandwell
500-599 North Division - Wolverhampton
600-699 North Division Works Services
700-700 North Division School Services
800-899 South division Works & School Services
900-999 "Services with special characteristics" (Limited stop,
supermarket free buses, race day specials, etc.)

double checks Centro bus maps

They do have some "A" and "H"
There's only one "H" I think, the 406H so numbered because it was
diverted some years ago to pass a Sandwell District General Hospital.

suffixes and "C" for clockwise on circulars and "N" for night buses.
I'd forgotten about "N". There are so few of those left!
--
Ian Jelf, MITG
Birmingham, UK

Registered Blue Badge Tourist Guide for London and the Heart of England
http://www.bluebadge.demon.co.uk