There is a photo in a Palo Verde Valley Times 1961 newspaper featuring the dismantling of the bridge. The main reason was to make room for the widening of the bridge (or a span as it says) in the future. Also in the caption, the local Chamber of Commerce asked the highway department to save the bridge for "foot or equine travel", but that got denied unfortunately.
Definitely looks like the Blythe bridge to me. I've found that postcard labels don't have a super great record for accuracy...I'm puzzling over a bridge right now in Maine with the same problem.