Originally Posted by Qtx

MI1 (Codebreaking), MI2 (Russia and Scandinavia), MI3 (Eastern Europe), MI4 (Aerial Reconnaisance), MI8 (Military Communication Interception), MI9 (Undercover operations), MI10 (Weapons analysis) MI14 and MI15 (German specialists), MI19 (PoW debriefing), MI17 (Military Intelligence "Head Office")

Some were active during 2nd world war, some got merged with other numbers.