'Named' exchanges (like my old one, WORdsworth) went with the introduction of all figure dialling in the late 1960s - the other change at that time was the introduction of the 01 for London prefix. All as I understand was to add more numbering capacity - both locally (limit to the number of names they could devise to match the possible numbers, especially if some degree of meaningfulness was desired) and nationally, as STD (subscriber trunk dialing: something we now take for granted, but then a novelty) came in. (As an aside, would you dial WORdsworth correctly?) The first of a series of changes to make more numbers available... whether there are enough to get through the century, we shall see And also, as international telephoning (and dialing) came in, the British number-letter correspondence, on our dials, came up against other countries' correspondences, in their numbers (and vice versa) causing confusion and wrong numbers... So once all figure dialing was established, by the 1970s, letters were dropped from dials. Only to reappear when we started to get imported phones, with all sorts of letter/number matching... But one of those questions - did we Brits get the habit of saying 'O' for '0' from the phone - or was it the other way round, that the GPO took it from what we already did?