The 8,431 foot high Teton Pass lies in the Teton Range of the Rocky Mountains within Teton County, Wyoming. Presently Teton Pass is crossed by Wyoming Highway 22 and Idaho State Highway 33. At one point the highway over Teton Pass was signed as US Route 20 Alternate. US Route 20 Alternate was over Teton Pass never formally approved by the American Association of State Highway Officials nor has the corridor ever been officially part of a US Route. The image above was taken from the 1949 Rand McNally Map of Idaho, Wyoming and Montana which shows US Route 20 Alternate branching from US Route 20/US Route 191 near Sugar City, Idaho and crossing Teton Pass towards Jackson, Wyoming.
The history of US Route 20 Alternate over Teton Pass
No major Auto Trail was ever assigned to Teton Pass as evidenced by the 1925 Rand McNally Map of Idaho, Montana and Wyoming. On the Wyoming side Teton Pass can be seen as part of Wyoming Highway 25 ("WY 25") whereas no State Highway is shown to exist on the Idaho side.
Following the creation of the US Route System during November of 1926 there appears to have been shifts in the State Highway designations in both Idaho and Wyoming. The 1927 National Map Company Sectional Map shows Teton Pass be reassigned as part of WY 22 whereas as no State Highway designation is shown to exist in Idaho.
The 1931 Clason's Map of Idaho shows Idaho State Highway 33 ("ID") assigned as a new designation towards Teton Pass and the Wyoming State Line.
Originally US Route 20 ("US 20") terminated at the east entrance station of Yellowstone National Park in Wyoming. On October 30th, 1940 Idaho Department of Public Works acknowledged in a letter to the American Association of State Highway Officials ("AASHO") that US 20 had been extended through West Yellowstone via multiplex of US 191 to Sugar City, Idaho. The AASHO description of US 20 had it jog west via ID 28 to Sage Junction to reach US 91. The Idaho Department of Public Works requested US 20 be clarified to multiplex US 191 to Blackfoot (through Idaho Falls) to reach US 91 due to ID 28 not being constructed to Sage Junction.
The 1944 State Farm Highway Map of Idaho shows US 20A branching from mainline US 20 at Sugar City, Idaho towards Teton Pass near the Wyoming State Line via what had been ID 33. From the Wyoming State Line US 20A is implied to descend Teton Pass to Jackson. It is unclear when US 20A began being signed after mainline US 20 was approved to be extended west of Yellowstone National Park.