MPSB meaning in Governmental ?

Special Branch is a label customarily used to identify units responsible for matters of national security and intelligence in British, Commonwealth, Irish, and other police forces. A Special Branch unit acquires and develops intelligence, usually of a political or sensitive nature, and conducts investigations to protect the State from perceived threats of subversion, particularly terrorism and other extremist political activity.

The first Special Branch, or Special Irish Branch, as it was then known, was a unit of London's Metropolitan Police formed in March 1883 to combat the Irish Republican Brotherhood. The name became Special Branch as the unit's remit widened to include more than just Irish Republican-related counterespionage.