This is an example of an impossible object, so called because there appears to be no way to separate the two pieces, or even put the two wooden pieces together in the first place. There is a dovetail joint on all four sides. Presumably the box is a take apart puzzle because there is something that shakes inside. It is possible that this has to be manipulated into a position that allows the two pieces to be separated. Unfortunately the pieces have probably warped, making them stick together so tightly that the solution for a puzzle of this type does not work. The curator of this website would be very pleased to hear from anyone who is familiar with this puzzle and knows the secret of how to separate the two pieces.