10/31/41 the US Highway Association No. 381 pitched the idea of US 381 from Oklahoma City to Yellowstone
Pg. 38 of the 2nd pdf at https://drive.google.com/drive/folders/17X1urAiDbTddFAvmltIwCxknVDArJhvY
State Highway Commission agreed to study it...haven't found further evidence of US 381 getting an official endorsement for AASHO request up through at least Jan 1946.