M2 .50 BMG TOMBSTONE DRUM

Need information on how the 200 round M2 tombstone ammunition chest, or drum, for the .50 cal. M2 BMG, is loaded? Does the belt wrap around the spool or lay at the bottom of the can and loop over the spool? What is the spring loaded bar at the top inside of the can for? Were these chests used with cloth belts, metal links, or both? Any information will be appreciated.