Nope, Fords have it too, that's actually where I saw it first - I was under a cab-chassis F350 with a 12ft flat bed taking off a steering damper, when I noticed what looked like plates welded to the inside of the frame rails, upon closer inspection it's actually a second pair of smaller frame rail pieces riveted to the regular ones on the inboard side. I remember wondering how hard it would be to find a pair of these and adapt them to my RCLB truck...
what you seen was 2 pieces of C-channel welded to the inside of the frame and 2 pieces of ford frame welded to the outside cuz the frame was stretched.cab/chassis trucks only come in 2 different cab to axle measurments,64" and 84".anything longer requires a stretch.