Photo courtesy of Rich Piehl (taken December 30, 2002)
Missouri isn't one of those "square" states, at least as far as outlines on state highway markers are concerned. But these markers on old Missouri 21, now a county road in Jefferson County, must have slipped through.
These signs were first reported in November 2002 by John Brocato in misc.transport.road. Rich Piehl took this and other photos a month later.
This sign assembly marks the approach of an interchange with the new Missouri 21, completed in 2002. Missouri 21 isn't a supplemental road, contrary to what these signs may indicate! Even if these signs were for a supplemental road, the rounded edges of the outlines would be unusual as well.