You got my attention, so I looked at my 65 Fastback (May, San Jose) but from the trunk (another Fastback is parked along side). Could not see enough from the trunk so that will have to wait.
But I have a theory. All 65 Fastbacks had a bumper as part of the latch assembly on the RH side. In 66, the bumper was eliminated on early Fastbacks but returned around Feb 1, 66. The bumpers on the front edge of the folding section were changed from a 1/4 inch thick, 1 inch dia, screwed in to the frame near the end style to a 5/16-18 screw threaded bumper a bit closer in sometime in the late 65 model year. The mounts in the fixed section were changed from the multi-angle bracket design to a oval shaped disc with a "cup". The fixed section had a "dish" in the sheet metal to accommodated this "cup". The used the existing long retaining screw and added a shorter screw to mount the "cup". This is an indication that Ford was concerned about noise from the rear fold down seat. The Shelby Forum has a few post on this subject.
What if the 65 Fastback engineering design concept was that the latch mounted bumper and a single folding section bumper and multi-angle bracket would be sufficient? When the first production models were made it was determined that the single bumper/bracket was insufficient and a second bracket/bumper was needed. The simple fix was to use the same multi-angle bracket on the other side with a pair of heavy duty 1/4 dia sheet metal screws and add a second bumper to the folding section front edge. That would explain the two different mounting styles, nutserts on one side, punched holes on the other.
Comments?
Jim