Previous to that most roads were a series of designated trails, such as the Lincoln Highway or the Dixie Highway. What's now US 71 was, in part, part of the Jefferson Highway
Then, in 1926, it officially became U.S. 71
Then in 1948, another style was added to keep up with the demands of high speed traffic
Then in 1961, the design was changed once more to the now familiar black and white
But there would be one more change: in 1971 the shield shape was changed slightly
Now if you really want confusing, check out Florida's "Kodachrome" signs :
Yes, I know: 71 doesn't go to Florida. But can you imagine trying to keep up with all these colors?
Oh, and one more:
City Route US Highway sign. Missouri used these as did several other states, but Arkansas did not.
No, that's not all. There are several other variants, but most are a combination of the above. Maybe I'll tell you about them, someday. ;)