The supposed stun grenade, or flash-bang, looks like a fragmentation grenade. Actual stun grenades have casings made to remain intact during detonation, containing most of the explosive force and avoiding fragmentation injuries, while large cutouts allow the light and sound of the explosion through.
The grenades being used by the pirates, which are supposed to be flash bangs but look like MK-II fragmentation grenades, are shown detonating with the striking lever still attached. Both flash bangs and grenades have a striking lever held in place by the pin, once the pin is removed the only thing holding the lever in place is the person's grip. Once released the lever flies up and off of the grenade, in the process the lever strikes a plate which ignites the fuse.