Just a couple of years ago I had an oak carrying case, hinged to open up like a book to reveal the interior which was fitted with alphabetical marked cubicles sized to these very cards. Mine had a set of similar cards relating to my home state. The case had a metal nameplate with the manufacturers name, patent date, and the following information: "U.S. Post Office Postal Clerk Test Unit".
Apparently this was how the government tested the skills of their postal employees or hopefull job seekers at one time.