14 feet seems a little narrow for a 1954 bridge too.
I agree with you that this truss is a little light for a 1954 construction date. It is also different than any standard drawings I looked at for the era.
I am going to speculate that the truss was designed by the Bureau of Public Roads in the 1920's and was relocated from somewhere in Southern Oregon to Cobleigh Road.
Iíll continue looking into it and get back to you if I find anything.
This bridge seems to be unique among the Oregon trusses... Do you have any thoughts about the construction date? 1954 seems too late to me, based on the lightweight structure and tiny gussets.