The order of the addresses was linked to the Coloured Books and needed translation to interwork with SMTP, and eventually the users needed to change their addresses. The point being, that anyone involved in UK scientific computing will have been aware of the UK protocols - many will have been involved in their creation and maintenance.
The humour comes from a (non-risqué) double entendre.
I don't think we'll agree here. To me this just looks like a straightforward use of a common metaphor for an address lookup that isn't funny to me whether or not it was intended to link it to the coloured books.
It still reads to me like just a synonym for "address book" to refer to a mapping.