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Since birdcages are not something you usually see sitting around, most of us know what they look like in a general term. As such, little differences are not obvious to most.
However, since there is a door beam added in '73, I suspect that the cage was reinforced in the rear door support and the front hing area to hold the extra load from an impact.
Your best bet is to find some that have burned where the cage is visible, take lots of pictures, and compare.