Side-by-side on the north service road of Interstate 70 at Missouri 79 in O'Fallon are two different types of Missouri markers. They don't look different at first, but examine them more closely.
If you look carefully at the northwest corner of the state's outline on the left marker, in the Platte Purchase area of the state, you'll notice that it shows the state's border as a straight line, instead of showing the indentation near St. Joseph as do most other state markers. The Bootheel is also different from the usual state highway marker. I call this a pointy marker. Other examples can also be seen in Mexico at the US 54-Missouri 15-Missouri 22 interchange, near New London at the US 61-Missouri 19 interchange, and in Kansas City on Missouri 1 just north of Interstate 35.
The marker on the right, by way of contrast, is a typical Missouri state marker. The fonts on the two markers are noticeably different as well.
Also subtly different are the arrow signs. The left-arrow sign is narrower than the usual Missouri arrow sign. In all other Missouri sign assemblies I've seen, all arrow signs are the same size. Maybe this one dropped in from Iowa?
Photo courtesy of Rich Piehl (taken March 2001)