By 1937, MT 1 had been concurrently numbered as US 10, and later that year, US 10 was rerouted, and MT 1 was renumbered as US 10 ALT. Also by that time, the route had been rerouted so that the overall length was approximately , and the entire length of the route had at least a graded gravel surface. By 1948, the portion of the route that traveled from Philipsburg to Drummond had been paved. In 1951, the entire length of MT 1/US 10 ALT was paved, with an overall length of approximately . Also, the portion of MT 1/US 10 ALT traveling from present day mileposts 9.426 to 9.940 was reconstructed along its current route. In 1957, the sections of MT 1/US 10 ALT traveling from modern route mileposts 16.757 to 21.934 and 30.703 to 38.405 were reconstructed along the present route. In 1960, the portions of the highway traveling from current mileposts 21.935 to 24.189 and 38.406 to 48.106 were reconstructed along the present road. The next year, the portion of MT 1/US 10 ALT traveling from the junction with S-441 to the junction with Elm Street (U-203) were rebuilt along the highway's present location. In 1964, the portion of the road traveling from the southern terminus to the junction with S-441 were reconstructed to the present day routing. Two years later, the portion traveling from modern milepost 62.323 to the northern terminus were reconstructed. In 1975, the portion traveling from Elm Street to Sycamore Street (U-201) was reconstructed to the modern route. In 1976, the route had an approximate length of .