Was this coating added by previous owners. Did the factory/ dealer offer it. Looks to be a black asphalt type of stuff

 

My 1979 F150 has a similar coating inside the doors.

The truck was undercoated by someone, not sure if it was the dealer or aftermarket, probably when new. There are plugged holes in the usual cab rust point: cab corners, front hinge posts. Front inner fenders done on both sides, entire cab floor & mounts, bed floor.