The Special Investigation Branch (SIB) is the name given to the three detective branches of the British military police, the Royal Military Police being the one most associated with the SIB and having the largest SIB of the three. SIB investigators usually operate in plain clothes but wear uniforms when travelling overseas.
Cormoran Strike was in the SIB, Royal Military Police, from when he quit Oxford University until roughly a year and a half before we first see him in The Cuckoo’s Calling. While in the SIB, he spent time in Afghanistan, Angola, the Balkans (including Bosnia), Cyprus, Germany and Iraq (his first assignment). He earned a life-saving medal for an incident that has not yet been revealed (It’s not the Anstis-saving incident). Strike was asked to stay in the SIB, even after he’d lost the bottom half of his right leg when a Viking exploded around him, but he decided to leave the Army at that point. After spending a month in a hospital bed, Strike then set up his private investigation agency on Denmark Street, London.
Very early on in The Cuckoo’s Calling, Strike gives Robin Ellacott the password to his computer, which is ‘Hatherill23’. A possible inspiration for the name is the Detective Chief Inspector George Hatherill, who recommended forming the Special Investigation Branch in 1940. He is also known for his investigation of serial killers John Reginald Christie and John George Haigh, and also the Great Train Robbery in 1963. It’s apparent why Strike might idolize Hatherill and use his name for his computer password.
The Royal Military Police have nicknames such as ‘Redcaps’, (since they wear red caps as part of their uniform), and ‘Monkeys’, which we assume is why someone over the phone calls Strike ‘Monkey Boy’ in The Cuckoo’s Calling. The Royal Military Police’s motto is ‘Exemplo Ducemus’, which translates to ‘By example, shall we lead’.