WW2 Military Police uniform

[Mandolin]

Hello! Its my first post.
I thought I might share this photo of my granddad, Augustus Savill, resplendent in his Military Police uniform during WW2.
I'm not entirely sure where he served; out East somewhere - there were mutterings of India but I'm not sure that's correct. He had been a plasterer before the war, but was very proud to have been a Redcap, and seemed to have enjoyed his time serving. He died in the 1970's.
I was wondering what might have been at the end of that chain - a whistle, perhaps? And whats the hooky thing for in the button hole above?