My Theory is that Engle, while working with Lincoln, was addressing continuity of style themes. Cadillac was very good at that, Lincoln was all over the place. Imperial, starting with the '64, being very different from the Exner era cars adapted past Imperial styling cues to maintain some continuity. The split grille was shaped like the 55 & 56, the spare tire bulge reminiscent of the trunk lid pressings, the ellipse from the 1959 rear bumper. The '59 elliplse in the rear bumper appears to be an evolution of the split inserts from 57 & 58.