A couple of years ago, I got my hands on a rare set used internally by LEGO. Apparently it was produced for the Production Planning department of LEGO, so probably only a few copies exist.

When I got it, it was just a small cardboard box including bricks, two stickers and a piece of paper which can be seen here. The box and the brick bag is just generic items that I added to store the set in.

As an extra goodie, the box included a special 2×4 prototype brick with thick walls, without stud logos, a round side pip and only the number “15” inside. I consider this a promotional item given with the set more than the brick actually being part of the set.