"Ello, ello, ello, what's all this then?"   
It's a rather splendid vintage sterling silver Policeman's Helmet charm by the well-known charm makers Nuvo, that opens to show a typical 1960s Police car.  Straight out of Z-Cars, perhaps.
There's a nice badge detail on the front of the helmet and it closes securely.
 
Marked for silver, it's in very good condition.
Comes with a jump ring ready to attach to your bracelet.