Definitions by CMB
box 850
Box 850 London SE1 was the postal address for SIS - the Secret Intelligence Service aka MI6 when they were based at Century House, 100 Westminster Bridge Road, London SE1 from around 1966 until 1995. They then moved to the current building at Vauxhall Cross.
craccin
It is spelled differently from crackin, although still having the same meaning, because the original word has the letters "ck" standing for crip killer.
"What's craccin cuz?"