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According to http://www.welovecorvettes.com/c3_models.html

The 1970 - 1972 Corvettes were the last to feature chrome bumpers front and rear.
In 1973, due to front impact legislation requirements, the front bumper was changed
to a body-colored flexible plastic. In 1974 the rear bumper followed suit.

How to help determine the model year?

http://www.oocities.org/motorcity/downs/3000/differyr.html
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