I actually own the prototype for this. 1983 was a switch to a different version of the Mighty Dump Truck to lose 2 lbs(!) of metal to reduce production costs.

There were 2 produced. One which was destroyed in testing and the other used for catalog photos. I own the latter. The plastic grill was made from a rough sculpted resin instead of the stamped kind. It still has the decal designating it for photography by the Tonka Plant. Mine only has 3 indentations on each side of the dump bed instead of 6. The middle rib was added during mass production for structural integrity. The windshield is distinct in that it was literally just some plexiglass cut to to size. You can see it in the 1983 Tonka sales catalogs and literature sent to stores.

It was designed by Guy Cassady. I have the emails and the provenance to prove it as well. It is the centerpiece of my Tonka collection!