Thanks for the clarification, guys--perhaps I need to get out to this part of the nation to see its historic bridges and get a feel for the differences in highway design standards between states that were used throughout the golden age of bridge building! Most of the '30's and newer bridges in my state of Minnesota are made up of straight, rolled beams and feature chords with either lattice (not V-lacing) or batten plates to secure the open side. The 1958 Lowry Avenue Bridge, which was demolished in June 2009, was an example with the lattice. I will keep my eyes open for these details. Thanks again!
Here's a boxed/punched bridge from 1936:
I agree that the 1939 date is correct.
For us Midwesterners, Michigan's Blue Water Bridge (1938) is another early example of that "boxed and punched" type of member: http://www.historicbridges.org/truss/bwb/index.htm
There are many trusses built in the late 30's especially in Washington State that have these same characteristics. The punched plate members and the heavy curved portal members are both commonly seen. The date on this bridge is probably correct, since there are other bridges that are similar in design and are pretty narrow for "modern" bridges in Washington State.
In Oregon as early as 1936 you can see these kinds of members on some of Conde McCullough’s last bridges. The Conde McCullough Memorial Bridge in Coos County, Oregon is a good example of this.
A good place to compare the late 30's vintage to the 50's vintage is Wahkiakum County, Washington. The Elochoman River Bridge was built in 1955, while the Grays River Bridge similar to this bridge was built in 1939. There is also the Chehalis River Bridge in Lewis County, WA that would be another good example for the 1930’s vintage.
I assume that the 1939 build date came from the NBI, but I somewhat question the accuracy of it, looking at the members used to make up this bridge: boxed-and-punched members weren't widely used until the late '40's through about the mid-sixties, when new truss bridge construction pretty much disappeared altogether. Also, it completely lacks built-up members, which indeed did start to fall out of fashion by the late 1930's, but were still used to some extent. The curved lower horizontal sway bracing members are also quite modern-looking.