After-the-fact Historical Marker
...AKA "Tombstone"
Tom Barrett at ODOT was kind enough to clear up the mystery. The historic bridge was indeed demolished and replaced. I include the MOA Tom sent as well as photos Tom provided of the interpretive signage.
This article appears to contradict my earlier finding (and that of the historic bridge inventory) that the bridge remains historic, and instead suggests all truss materials are new, bolted, and galvanized. http://www.galvanizeit.org/project-gallery/gallery/washingto...
It was open to traffic as of October 2013 when I drove by.
I don't think this bridge was demolished. From Historic Bridge Inventory:
The 3 span, 267'-long bridge is a continuous, Warren deck truss. It was rehabilitated in 1999 with replacement of the original deck and railings.
...I got a marker for them!
Thanks Todd Baslee as always for the pic of the loo