MI5 & MI6 but what are the others MI3, MI9 etc

[TonyGosling]

MI1: Director of Military Intelligence, also Cryptography
MI2: Responsible for Russia and Scandinavia
MI3: Responsible for Germany and Eastern Europe
MI4: Aerial Reconnaissance and maps
MI5: Domestic Intelligence and Security
MI6: Foreign Intelligence and Security
MI7: Propaganda, Deception Planning and Psychological warfare
MI8: Interception and Interpretation of Communications, Signals Inteligennce, GCHQ
MI9: Clandestine Operations, Escape and Evasion
MI10: Weapons and Technical Analysis
MI11: Field Security and Police
MI12: ?
MI13: ?
MI14: Unorthodox secret communications: D section, pigeon message service
MI15: ?
MI16: ?
MI17: Secretariat for MI departments
MI18: ?
MI19: Prisoner of War debriefing unit

https://www.theguardian.com/notesandqueries/query/0,,-82187,00.html
http://www.mi5.gov.uk/output/Page255.html

[Gridkeeper]

MI18 - Occult Branch.