I read about this in the Unofficial Microsoft Office Stuff blog:
What is original fifties clipart?
Most communities in the fifties had small town print shops that doubled as printers of local news and advertising papers. These printers could not afford graphic artists so they used stock clipart supplied by large companies who distributed common graphics for use in advertising sections of the papers. They were provided for the printer in lots of categories to meet any advertiser’s needs.
Check it out at http://www.retrographix.com/