Two-Piece Locking Puzzle

A puzzling puzzle ;-)
The idea, is, of course, to separate the puzzle parts.
Even if you know what's inside, it might be a slight challenge ;0)

#Printing and assembly

##Prepare:
You will need:
4 metal (I used steel) balls about 9mm in diameter (slightly smaller shall work too)
4 pieces of metal rod ∅4mm (I used brass rod), each about 28-30mm long.

Sand rod ends so that they are rounded. Best done by chucking the rods in a lathe, but an electric drill will do nicely too.

##Printi