I recently looked at Rand McNally map dated to 1949 which shows US 20A over Teton Pass. I've never seen an alternate US 20 on any other map nor is as there anything pertaining to in the AASHTO database. Does anyone actually know if Wyoming and Idaho were actually signing US 20A as a continuous Route bypassing Yellowstone National Park?
http://www.davidrumsey.com/ll/thumbnailView.html?startUrl=%2F%2Fwww.davidrumsey.com%2Fluna%2Fservlet%2Fas%2Fsearch%3Fos%3D0%26bs%3D10%26lc%3DRUMSEY~8~1%26q%3DMontana%2520road%25201949%26sort%3DPub_List_No_InitialSort%2CPub_Date%2CPub_List_No%2CSeries_No#?c=0&m=0&s=0&cv=0&r=0&xywh=14022%2C3756%2C2377%2C3893
I assume the point of that was a Yellowstone bypass essentially via what's now SH 33/WYO 22 and US 26. Which, interestingly enough, is quite a bit shorter than following US 20 between Sugar City ID and Shoshoni WY. In an alternate universe, maybe US 20 would have gotten rerouted that way, especially since almost all of its existing mileage between those points was concurrent with something else.
That map would suggest this Alt US 20 was not a thing in Wyoming, which of course would make it useless as an "alternate" if it isn't signed all the way through. I almost wonder if this was just some sort of plan to get Driggs and/or Teton Pass on the US system.
US 20A also shown on the reverse of at least one Wyoming official. Mentioned in the 1949 Wyoming application for US 26 in the AASHO database.
Also, Wyoming asked for the reverse of what the map shows. They wanted US 20 via Teton Pass and change exisitng US 20 to US 20 ALT. See the 4th pdf here - https://drive.google.com/drive/folders/17X1urAiDbTddFAvmltIwCxknVDArJhvY - and go to pp. 56-58.
Condensed this to blog form, appreciate the feedback above towards figuring this out:
https://www.gribblenation.org/2021/11/paper-highways-us-route-20-alternate.html