In summary, and from Wikipedia: Director_telephone_system:
Quote:
The three-letter code eg WIMbledon was written in bold capitals if the caller should dial all seven digits. If written merely in capitals eg REGent it indicated that the desired number was on an exchange which had not yet been converted to automatic working, and that the caller should dial only the initial three code digits, and wait to be connected by an operator.
Later some of the remaining manual exchanges were equipped with Coded-Call Indicators (CCI) which displayed the local digits dialled by the caller to the operator, The number would be listed as for an automatic subscriber with the first three letters in bold, and automatic subscribers would dial all seven digits.
If an exchange was outside the Director linked-number area, the exchange name and number was in small type, e.g. Laindon 2263.
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I don't think there were any (public) automatic exchanges in the London Director Area before the Director system - the problem was that such a high proportion of calls would be non-own-exchange there would be little benefit in using an automatic exchange until there was a reasonable prospect of having all of London go automatic.