SH-6 was extended to the south on July 7, 1975, bringing it to Altus by way of a concurrency with US-283, where it joined US-62 in another concurrency, headed west. West of Altus, the route split off and headed southwest to the Texas state line. In addition to the U.S. routes, SH-6 was concurrent with SH-44 between that route's current southern terminus and Eldorado, where it ended; thereafter, SH-6 followed SH-34 to the Red River. To remove the redundant designations, both SH-34 and SH-44 were truncated to their current southern terminus on January 5, 1987.