Definitely printing 'slugs' from the long ago lead type era, which went by the wayside about 40 years ago. These days, offset printing plates are thin sheets of aluminum, coated with a photographic emulsion. The standard for the past 20 years or so has been 'direct to plate', which means the page is laid out on the computer in a desktop publishing program like QuarkXPress or InDesign and then printed to the plate that goes on the press. It's a pretty cool artifact, though.