This is a machined aluminum 'flash bang' grenade (it came in a zip-lock bag that was labelled 'flash bang'). I don't remember seeing this particular type of grenade in the movie though. It features a removable safety pin and a spring-loaded safety lever that will actually fly away like on a real grenade. The top and bottom screw off so something could be placed inside to produce whatever effect the production wanted.Here it is with the pin and lever removed.
