After tinkering with the shape of the puzzle, I returned to the classic 3 x 3 block configuration, but introduced a new, fourth stamp. This stamp is similar to the third stamp, in that it doesn’t hold the special properties of the first two stamps.

As my experience in making puzzles increased over the years, the complexity of the rubber stamp puzzles increased as well. Comparing the detailed solution of this puzzle and early 3 x 3 puzzles, such as Puzzle 2 or 3, you get an idea of how the construction of these puzzles has evolved. Instead of one challenging step, there are many.