US 163 was commissioned by AASHO* at their June 1970 meeting. The designation ran from Kayenta AZ on the south up to Crescent Junction UT on the north (illustrated by the green lines on this map):
A similar proposal was denied in June 1972. The only difference was, instead of splitting into east and west branches, US 163 would have simply followed the east branch (blue line on the map above). But not all was lost: about ten years later, AASHTO* approved a proposal to incorporate much of this corridor into an extension of US 191. More info can be found on the main pages for US 163 and US 191.
*AASHO became AASHTO in 1973.