This image is originally from the 1928-1948 Ford Chassis Manual, otherwise known as the "Green Bible" to early Ford enthusiasts.  These diagrams gained a lot of popularity in the early Victor Page manuals because they were helpful for the view to see and understand the complete assembly of a part of the car from a single image.  In this image, we see the 59AB flathead engine.  From the location of parts in the image and the numbered tags we are able to discern three very key pieces of information.  The first is the part numbers: each part is individually labeled with a number corresponding to the numbers in the parts books.  This is key because it identifies parts visually for people that may not know the factory name for them.  Second is that is shows the parts in the order in which they are assembled.  This is helpful to the builder who is not sure in which order parts need to be assembled.  Lastly, it shows the final, completed product.  By showing one side of the engine 'exploded' and the other side assembled they are able to illustrate what the final product should look like.