The inner part looks like a large sprocket. The rounds are held on the outside edge against the inside of the drum. The coil spring is wound around a post on the 'sprocket' and a tab on the drum.
I had to lightly file the outside magwell portion on mine to get it to function properly. There needs to be a bit of play when it is inserted. If it locks up too tightly, FTF will occur. Easy to fix.