I do not know the exact history of the Revenue Bridge but here is what I do know.
The bridge has been rumored to have been moved in from another location which is very popular practice for the Clackamas County Public Works Department.
I am not sure if the structure started as a through truss somewhere else, but it does have stout cross beams on the bottom chords of the trusses. This leads me to think that your idea is very resonable considering the past practices of Clackamas County.
I have actually been in search of the history of this structure since it should be gone before the end of the year.
Do you think this bridge started life as a through type bridge and later it was recycled and placed here in its present configuration?