factory undercoat question

My 340 Swinger was built 1/22/1970. Code J55 on build sheet, undercoat and hood pad. I removed a small piece of undercoating clear in by the driveshaft tunnel. It was same as body color FY1, under the black floor saving substance. I firmly believe that is why my floor is in such good shape.I've got 2 '69 340 Darts neither undercoated both Barney Rubble models,, the floors are gone.