The 1912 California State Automobile Associate Tour Book does not show any of the route through the Mojave desert. The main route at this time up until about 1914 or 1915 was the "Ocean-to-Ocean Highway" route east into the desert, then southeast to Brawley and Yuma. The other option was to follow the Coast Route south to San Diego, and take the auto trail east through Campo, Jacumba, and on to El Centro and Yuma.
However, an article in the 1912 Touring Topics included a map #92. It actually has a 1912 copyright notice, a practice which ended in 1918. This is the oldest map I've found for the route. Notes follow the image.
This is a pretty small scale map, but a few points can be noted:
The highway did not seem to cross the rail line at Verdemont, but instead at the actual hamlet of Devore right at the railroad. This will be seen more clearly on the 1919 TIB map. The road seems to cross west of the railroad then back east near the Blue Cut area. It crosses west again at Cozy Dell. There is an 18% grade shown near the summit, so on this map it seems the road
may have taken the old summit road at this time (Forest Road 3N45). This map show the diagonal alignment to Hesperia, only a trace of which remains today. The old road via Stoddard Well to Daggett is shown, and this was not only shorter, but a very easily drivable road, much of which still exists.
