Not necessarily on the E numbered cabinets, there are some E1, E2 cabinets that have been in place for many years as a standard PCP, i.e. a great variety of naming conventions do exist

Also where exchange only lines are converted I cannot think of one instance where the new cabinet has been called Ex (where x is a number), the usual current convention is to use the next free number e.g. if an exchange has 6 cabinets (P1 - P6) then it would be P7.

Well there you go.

As an example, on the Vauxhall exchange our EO bundle of 100 lines went from being Exchange bundle 70 something to AIO cab WRVAUX P54. 54 being the next free cab number.