Sorry to be the "fly in the ointment", but I do question or doubt that it was "standard practice" that the oval holes were modified, That is not to say that some oval hole "left over" inventory may not have been (although if there were left over oval plugs, why would the holes have been modified ?). The reason I say this is because of several things. In the pictures that Bob G just Posted, as well as the one that John posted with the duct tape "background" in his summary, note the axis of the supposedly remaining portion of the oval holes; it is vertical (this is also what is shown in the Illustration in the Body Assembly Manual, page N7-8105-1, for the square holes). The oval holes in the radiator support had a horizontal major axis. And finally, the rubber plug (Part # 376966-S) shown in Bob G's most recent Post is not an oval plug, but a 5/8" diameter round button plug according to the Ford Standard and Utility Parts Catalog.
I truly feel that the square hole was a totally new stamping. The square part of it used for the Cougar headlight applications, and the two rounded (segments of a circle) portions in order to facilitate the use of a round, button plug. The oval plug (Part # 377841-S) is totally different than the button plug.
Based on the Body Assembly Manual revisions, along with all of the pictures that have been furnished, I think we are dealing once again with another Production Change. Oval holes with oval plugs early in the year, and square holes with button plugs from about mid-year on.
Bob