Business US 54 in Mexico runs along the former route of US 54 through the Audrain County city. When US 54 was moved to a route bypassing the city, Missouri 22 and 15 were also extended to meet US 54 just south of Mexico.
In this photo, taken in March 1998 at Boulevard and Clark Streets, note how the Business banner is mounted. It's set somewhat behind all the other markers. Perhaps the original assembly just had the US 54 marker without any directional banner.