Where did the state outline go? When a new route for Missouri 21 was built in Jefferson County, some of these signs appeared on old Missouri 21 two miles south of Otto, just ahead of an interchange with the new 21 (completed in 2002). Missouri 21 isn't a supplemental route (which all are designated with letters), so you have to wonder what happened with these signs.
The metal posts indicate that they may have been erected by the contractor that built the new highway -- or, at least, they are nonstandard signs. In this case, they're really nonstandard. This isn't just an isolated instance, either, at least at this interchange.
John Brocato first reported the appearance of these square-outline signs in misc.transport.road in November 2002. Rich Piehl took these photos on December 30, 2002.