I would not be surprised to learn that the original WJ Olds body may have recieved new **updated** Regal sheet metal at some point, along with a fair bit of technical assistance from GM.
Those G's ended up spending a lot of time in the big GM wind tunnel, as the competition potential became clearer. Changes so subtle, they would be hard for most people to pick them off.
(Template checking was an inexact science, then.)