Over here in the U.K. director SxS systems were employed in the six largest urban areas: London, Birmingham, Edinburgh, Glasgow, Liverpool, and Manchester. The directors were indeed between the linefinder and first selector, although they dropped out after the call was cut through so as to be available for other calls.
The directors could translate the 3-digit prefix into a string of routing digits to go into the local switching train or to select the appropriate outgoing trunk and then dial the necessary routing code(s) into that trunk. I forget the precise number now, but a prefix could be translated into anything up to a dozen or so digits if necessary. A store-and-forward register in the director then just repeated the last4 digits of the number after it had finished its routing translation.
The first selector (which was controlled not directly by the susbcriber's dial but by the director) had the levels arranged to make the most efficient use depending upon local traffic patterns. Commonly used outgoing trunks could be connected directly from the first selector so that the director only had to pulse out a single digit to select them. Less commonly used trunks would be placed on second, or even third selectors.
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