At the start of 1969, Iowa renumbered some of its highways. In many cases, the new numbers matched highways in adjacent states. For example, Iowa 60 was renumbered Iowa 5 to match Missouri 5, which then allowed Iowa 33 to be renumbered Iowa 60 to match Minnesota 60.
The Des Moines Sunday Register article that featured this map described the renumbering as "the most extensive revision of state highway numbers in recent years." Signs were changed beginning December 23, 1968, according to the article, with 1,300 new signs being required. Some signs were just moved from their old highways to their new routes.
In addition, some concurrences were removed, as indicated in a fair use excerpt from the article that's also shown at left. Most such route numbers that were removed were concurrent with U.S. routes.
Jason Hancock's Iowa Highways site also has an extensive listing of route changes in Iowa before and since the 1969 renumbering.
A new, better-quality scan of the map, plus the addition of an excerpt from the article covering the removal of concurrent route numbers, were added October 14, 2017.