My source is that I once leafed through a Japanese in a book store book for rural diy homesteader types that had detailed instructions on how to make a retaining wall for terraced rice fields not dissimilar to the one you pictured, and my impression is that there is a lot of engineering behind the surface, and also the arrangement of the sizes of the stones to make it stable and movement resistant (like to settle back into place after a quake), that make this more than just fitting the shapes together like a jigsaw.. I wish i had actually bought that book now.