I'm not saying that the pony seats are a fluke. I just question whether or not they were a Factory Ford option. "Factory" meaning it was installed while the Bronco was being built. My personal experience is that I have seen not seen any Ford documentation for the Bronco pony seats being offered but that doesn't mean it didn't exist.
Given the apparent lack of documentation in the Ford material I have seen, I believe the Pony Interior may have been a dealer option. This would be similar to the optional items that were ordered by Duane Kennedy back in 1967 at a Ford dealership for the Silver Fox. In the scan of the purchase order below you'll note Warn hubs, tire carrier (not a Ford Factory option until 1968), and fender cut-outs. None of these items were Factory installed items. Btw, you'll note the item "Special Paint" on the purchase order and the window sticker. The Bronco was ordered to have a non-standard Bronco color, in this case Thunderbird Gray. Duane told me that the color was applied at the dealership.
These items were not listed on the resulting window sticker.
Again, this is just an observation and "guess" on my part.
I am admittedly skeptical of stories of what is or is not factory based on my experience with talking with Bronco enthusiasts over the years. Believe it or not I have had conversations with folks that swore up and down that Ford offered the 351W as an optional engine. The proof statement given was to consider how many Early Broncos have them. That's just one example, there are been many other similar claims just like that one.
I look forward to the day that someone can actually produce documentation from Ford explaining the story behind the pony seats.
-Terry