Look at the heads of the special bolts that hold the brake backing plates to the end of the axle tubes. A standard 65 Skylark rearend has oval shaped head bolts, and a '65 GS rear end has round headed bolts that have a flat spot cut on one side. This also applies to the station wagon rear ends, but they are easily distingished because they have 2.5" wide rear brake shoes on them and have higher bump stop mounts welded on the tubes.
