I was 56 nin/ 23 war killed a sabretooth tiger and figured out coffer number. I also got cotten gaiters from sapling's box. Was skilling up GK while farming i was around F-7. What i did notice is killing the same mobs over does not get you a chest. From pure eyesight i killed a may fly, then sapling, then tiger or goblin and mixed and matched and normally always got a box. Happy Hunting