Map of US highway 61 and family, and US 218
US 61 is represented by the purple line beginning in New Orleans LA. Its current north terminus is in Wyoming MN; the red line extending north of there is the historic extent of US 61. Also shown in red is US 61's historic routing through Fredericktown MO.
US 161 is the red line from Dubuque to Keokuk IA.
US 218 is also shown on this map: the blue line beginning in Keokuk IA. Its current northern terminus is in Owatonna MN; the red line extending north from there to St. Paul is the historic extent of US 218. This is a north/south branch route misnumbered as if it runs east/west, so I included it on this map to demonstrate how the highway would fit into the US 61 family, if it were to be assigned a number US x61. This is just one possibility that makes more sense than 218; I've also included the route on the maps for US 63 and US 65. US 65 was the only one of these routes that connected with US 218 in 1926. But in 1938 US 218 was extended to connect with US 61, and personally I wish US 218 would've been renumbered as US 161 at that time. I think that would've been the best choice - not only since it connected with US 61 on both ends, but particularly because much of current US 218 follows historic US 161 (the red-blue dashed segment). You can also view the US 18 map, which shows US 218 with its implied family.