MO5(g)

MO5(g) was a specialized counter-espionage branch of the British War Office during World War I, primarily responsible for investigating and disrupting anti-British conspiracies and revolutionary movements. In 1916, this department was renamed and formally became known as MI5. 

The designation breaks down as follows: MO5: Directorate of Military Operations, Section 5.

(g): A sub-section specifically created to monitor the Indian revolutionary movement and disrupt plots by foreign agents attempting to incite rebellion against British rule in India. The "g" section ultimately grew and was reorganized into a broader counter-intelligence agency in 1916. This marked the birth of MI5 (Military Intelligence, Section 5), which operates as the UK's domestic security service today