"Front Splash Apron" is/was the name for the similar piece between the front bumper and the lower piece/pan on the grill.
"Stone Shield" or "Gravel Shield" are/were the names that are/were used for the "Rear Valance" - the similar piece that goes between the rear bumper and rear of the panel body trucks (standard equipment).
Most likely there was no standard piece like that for the rear of cab trucks because there was no standard rear bumper during Advance-Design years (just optional equipment or accessory or after-market rear bumpers), except on panel body trucks (Panel, Canopy Express, and Suburban).
