I've had a few roadsters years ago. I think what he meant was that some of the major parts are the same like rear body section & lid, doors and side aprons. The cowls and hoods do differ. Then it gets to things like fenders, and some are the same, but there was a very slight difference in very early fenders built in 28 and early 29. It's tough to tell them apart without both cars side by side.
I did spot a 29/30? 65? coupe MANY years ago that was a 5 window, and I have never seen another. Ply & DeS coupes were 3 windows. I took a polariod pic of it....it's somewhere here
Edit: The models I was mentioning are: 29 DeSoto 6 model K and 30 CK. 28 Ply Q. 29 Ply U. 30 Ply 30U
It sure looks like that 75 roadster shares the same doors, but it sure looks like there was a few inches added to the rear quarter panels between the door and rear fender. It looks longer there, plus the upper rear deck that has the top rests, looks longer as well.