I will look into this bridge, especially since this one is in my childhood backyard. I grew up east of Woodland near the small town of Yacolt in Clark County.
I am going to speculate that the bridge you have referred to that was built prior to the 1940 structure was actually located about a half mile upstream when the county road bridge is today.
My reasoning is that the original route of the Pacific Highway crossed the river at that location. The realignment of US 99 in the 1940's took the more direct route bypassing the unstable hillsides along the two Lewis Rivers and the town of LaCenter. I-5 follows this same route today.
I will look into it and get back to you.
There existed a previous highway bridge here, dedicated May 17, 1913. 2,000 spectators showed up. Arthur Cavill, a swim instructor from the Multnomah Club in Portland was seriously injured in a sack dive. While tied in a sack and dropped 65 feet in an attempt to perform a signature Monte Cristo dive he struck a wood crate floating in the water and was not resuscitated for over 30 minutes.