Re: London alphabetic telephone exchanges - is there a map?
In times-past it was quite common for businesses spread across several sites within ten miles or so to rent 'tie lines' from the Post Office (which in later times would have been called EPS8 or EPS25) - so people at the sites could place [through their company's outward-facing telephone-operators] calls to what were essentially 'remote' extensions.
As a result, you could be able to call the outward-facing operator at site X and ask for a number located at site Y - if there was nobody else using the 'tie-line' the operator could put you through. There were various 'unofficial' kludges done which allowed you to directly-dial extensions across tie-lines through such company switchboards from the outside.
I temember a company called "Telephone Rentals" who provided semi-automatic switchboards that enabled this, much to the annoyance of the GPO.
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