Beginning in 2000, Missouri gradually switched over to wider-style markers for two-letter supplemental routes, enabling the use of larger fonts on the markers. In this photo from April 2002, Route A is marked with the new, larger font while Route BB is marked with the earlier, smaller fonts, a square outline, but wide borders and a rectangular sign blank.
This assembly is on US 65 north of Branson.
Photo courtesy of David Backlin