Trucks are often first
One of the persistent oddities of the auto industry is that trucks are much more resistant to change, but trucks often got new features before cars.

I've always been puzzled by the fontanelle or soft spot in car roofs, which was finally fixed by GM in 1935 and later by others. Supposedly the obstacle was in the metal-forming tools. Forming a broad shape with pillars on the sides and an unwrinkled middle part was impossible with regular presses. It had to wait for a complex and sequential deep-draw press. Until the tool was perfected, automakers left the middle section of the roof open, and filled the space with a wood frame coated with leather or cloth. This was a serious problem for both maintenance and structural stability. Owners had to refinish or replace the cloth often, or suffer complete leakage.

I don't know anything about presses, so I can't argue with the basic necessity. But the wood and cloth solution is obviously a bad idea, and could have been done better from the start.

In the '20s, the open area was a one-dimensional curve, flat from side to side. A single piece of metal could have been cut and simply draped over the frame, then welded in or weatherstripped in. Later the area had a slight side-to-side curve, so a simple drop-in wouldn't have worked, but the piece could have been formed by an ordinary rolling mill without noticeable wrinkles.

After GM developed the deep-draw technique in '35, others scrambled to meet the advance. Chrysler and Hudson switched to a metal piece for the open area in '36 as a temporary expedient, thus proving that it was always possible.

Here's proof that the open area was never necessary on coupes.

A '35 Ford coupe showing the cloth part clearly.




A '33 International pickup, based on a Willys design. Clearly the roof is all metal, and the shape is pretty much the same as the '35 Ford coupe.



IH didn't have nearly as much money or staff as Ford or GM, and truckmakers were always focused on function instead of styling. Despite those limits, IH didn't have any trouble forming a single piece metal roof.

I think the key difference was the pillars. After the war Willys wanted to return to passenger cars but didn't have the bullypower to contract with the bodymaking firms, who were more than fully occupied with the Big Three. So they asked Brooks Stevens to design a station wagon that could be formed with the resources of a basic sheet-metal shop. The result had a long all-metal roof, smoothly curved in both dimensions. In both of these Willys designs the roof is a cap, placed on top of the pillars and welded. The bigger carmakers apparently wanted to have a smooth curve from pillars to roof, and were willing to sacrifice structure and durability for this one curve.
  ¶ 7:03 AM