Nick, I don't know about GMC's, but the '39 Chev. used the bumpers with the wide single groove, and the '40 used the bumpers with the double narrow grooves. '39 pickups used the same bumpers front to rear, '39 panel truck had a special rear bumper that "dipped in" in the middle to allow easy close-up access to the cargo area with the rear doors open. The pickups didn't use this special bumper, as with the tailgate down, you never got close to the bumper anyhow.
