"D" stood for Director - the lord of all he surveyed, an ATCO2 when that meant something! (like when I had to do a promotion board to get to be an ATCA2)
The D had overall control of his sector and the rest did his bidding - radar (an ATCO3) only separated what he was told to do and A (ATCO4) produced, by hand, all the strips and passed the estimates etc.
Best exchange: - "Radar, can you drop ABC through XYZ for me?"
"Sorry D not right now there's only 2 miles between
them"
"OK - I'll do it, I've got 5 minutes between them on the times!"