US 231 was included in the inaugural routes of Nov. 1926, but at the time it ran only from Montgomery AL to Marianna FL. US 241 was not among the original routes*, but neither was it far behind: it was commissioned in Mar. 1930. This was mostly done to appease certain Alabama interests who wanted a US route designation to be applied to their State Highway 1.
*For the sake of clarity, there actually was a US 241 that was commissioned in 1926 to run between Nashville TN and Hopkinsville KY. However, it was never signposted as such in Tennessee, and possibly not in Kentucky either. AASHO officially canceled the US 241[i] designation in Jun. 1929, and its number was recycled less than a year later and applied to the route discussed on this page. To avoid confusion, this route is sometimes differentiated as US 241[ii].
Gaps, US 20[e], US 20[w], US 441[n], US 441[s], US 287[n], US 287[s], US 89[n], US 89[s], US 191[n], US 191[s], US 63, US 66, US 10, US 311, Rouses Point, US 175, New Orleans, US 167, US 21, US 91, US 189[i], Jacksonville, US 92, US 151, US 79, US 270, Yellowstone NE, Cody, US 420, US 320, US 310, US 16, US 116, Rockingham, US 6, Covington, US 41, Brady, US 22, Bay Area, US 222