One of the stories I’ll tell in *How Comics Were Made: A Visual History from the Drawing Board to the Printed Page* is about a sheet of Doonesbury printing molds (called flongs) from 1973 that I purchased from eBay a few years ago. In fact, these strips are probably the precipitating act that led to this book—and the current Kickstarter campaign!
This is a sheet of six dailies (Monday to Friday) a syndicate would send out to newspapers (see book excerpt as image).
https://www.kickstarter.com/projects/glennf/how-comics-were-made