Here are some shots of Missouri route signs provided by David Backlin. These show some variations of the current shield design.

This sign assembly near Blue Eye shows a modern version (MO 13) beside a 1960s version (MO 86). The older one had a smaller shield and smaller text.

This sign is a modern borderless version using the square sign and B font.

These are of the current design using the square signs. The MO 76 sign uses C, and the MO 265 sign uses B.