About the Boxes:

 

The 12 Labors letterboxes are located in 6 different parks with 2 boxes in each park. Since the puzzles are independent from each other, the cluing for boxes in the same park is also independent. The stamp images can be assembled to form 4 Greek vases, each broken into 3 fragments. You might want to leave extra room in your logbook or make extra impressions if you wish to eventually have 4 complete vases.