I stumbled upon an interesting find today regarding early US 101/Redwood Highway from Hopland south to Cloverdale. It seems the original highway followed what is now Mountain House Road and present CA 128. This segment of the original Redwood Highway was completed by 1915 as part of early Legislative Route 2. The new (for the time) Russian River Route between Hopland-Cloverdale opened to traffic on February 21st, 1932 and was covered in the February 1932 CHPW:
https://archive.org/details/californiahighwa193233calirich/page/n56/mode/1up?q=Hopland
So essentially this would have been early US Route 101 southbound at the Sonoma County Line:
(https://live.staticflickr.com/65535/50854257071_318eb662d5_4k.jpg) (https://flic.kr/p/2ktPmTH)0 (https://flic.kr/p/2ktPmTH) by Max Rockatansky (https://www.flickr.com/photos/151828809@N08/), on Flickr