As far as I know, all the doors had the same # of door panel clip holes which would be 7 holes towards the bottom of the door. It's very possible that the 70 door panels used all of these holes, while the later door panels may have only used some of those holes, I'm not sure.

As far as the upper inner bracing not being as robust, I agree totally. On the 70 doors it is a single layer of sheetmetal, while the later doors use a double layer. No big deal as far as interchange goes, both would work.

The easiest way to spot a 70 door is by the oval holes the cat whiskars clips go into. The later doors use screws to hold the cat whiskars on. If you try to put a 70 door on a non 70 car, you'll need two sets of cat whiskars because the oval holes for the clips are right where the round screw holes should be, and you won't have anything to screw into. On my own car, I had a non 70 door on my 70 and used a dremel to make the screw holes oval so they would accept the 70 style clips so that I wouldn't have to buy two sets of cat whiskars.

I've heard that At some point, 73-74ish I think, the window regulators were changed from a plastic slider in the track, to some kind of roller bearing in the track, and that this new setup required a few more holes in the door for access to bolts for adjustments, etc. It seems there would be at least 3 different doors then. 70 only. 71-whenever the regulator change happened. After the regulator change.

I'll try to write more later. Still looking for those pics.

Tav