Until 2000, Missouri usually tried to fit the numbers for three-digit highways into square sign blanks. When the first number is "1", as on US 169 in the northern reaches of Kansas City, there's usually no problem fitting all the numbers in.
This marker is typical of markers used on freeways: note the absence of a white border and the use of a metal pole rather than a wooden post.
This photo was taken in October 2003.