And the term "plate" referred to a photographic glass plate that was used to print the page rather than a plate of set type. You physically couldn't do both on the same sheet of paper, or in some cases on the same type of paper stock. My copy of Penny Whimsy (1976 edition) has about 50 plates that the back of the book, on shinier paper with higher resolution photos than would have been possible on the paper stock of the rest of the book.