I don't know if it will help you find an answer but...Never.
You see,this one is a temporary casket/chest. You keep opening them,in the end you get this,but you can never open it,in another update,you lose this one and get another casket,when you open that casket. Bam! You find this again,just like playing with matryoshkas but instead of getting stuck with the smallest one,that smallest one gives a smaller every once in a while.
Hope it's enough to explain how this works.