The unusual Missouri 152 markers near Liberty also have these square counterparts, which appear on a side street. As with their rectangular counterparts, the outline tries to be unusually detailed about the shape of the state's borders.
Almost every other Missouri state highway marker uses one of two versions of the state outline (see here or here for a direct comparison), Yet this marker is not a construction marker; it's a permanent installation.
Also notice that the sign blanks have not been rounded off.
This photo was taken in October 2003.