Map of US highway 19 and family, with US 121
US 19 is represented by the purple line from Erie PA to Memphis FL. Across the NC/TN stateline (between Bristol and Asheville), the route is split into US 19E and US 19W. Originally US 19 followed the red line to Lawrenceville GA. Later, south of Thomasville GA, it followed the red line to Tallahassee FL, essentially along what is now part of US 319 (see below).
US 119 is the blue line from DuBois PA to Pineville KY.
US 219 is the blue line beginning in Lackawanna NY. Currently its south end is in Rich Creek VA; the red line south from there to Princeton WV is the historic extent of US 219.
US 319 is the blue line from Eastpoint FL to Wadley GA. Historically, north of Thomasville GA, US 319 followed the red line to Wenona.
US 121 is also shown on this map (the green line from Beckley WV to Pound VA). Its number is a violation of US route numbering conventions, because it doesn't connect with US 21 (its implied parent route - see map). So I included it on this map to demonstrate how the highway would fit into the US 19 family, if it were to be assigned a number US x19. I think this would be the best choice, but US x23 would work too.