What's cool is that GM made a provision for the fourth door, but never was able to actually BUILD one. The reason (I've heard, anyway) was that in the '50s and '60s, as we all are aware, the only four-door utility truck was the International Travelall. The folks at IH actually trademarked/copyrighted the design (notice no one else had one, right?), and when the '67 GM trucks were in the design stage, talks were going on to where IH would license the four-door design to GM for "$X" per truck built. No agreement was reached, but in the interim, the GM four-door design went forth. So, the provision for the fourth door was there, but never put into production.
In 1972, when the all-new '73 trucks were being designed, IH and GM came to an agreement and the '73 trucks were produced in four-door models, and the three-door became GM history. Kaiser-Jeep's (then AMC) Wagoneer was considered a station wagon and was not affected by this.
Whether this is all true or not, I can't say with certainty. But, when you think about it, neither Ford nor Chrysler ever produced a four-door Bronco or Ramcharger (both existed then), and only GM, IH, and Jeep built a four-door utility-style passenger wagon/truck in that time frame.
Ahhh, but such is not the case these days...
