55-59 rear cab differences

[bigblock73]

Hi all, need a little schooling on the differences between the 55-59 cabs. The area I am specifically trying to understand, is the area just below the rear window at the door striker location. I have a 58/59 (I believe) GMC big window cab, and as you can see, there is a curve at the inside cab corner adjacent to the door latch. I believe this does not exist on all cabs, as I have seen some where its a straight across body line without the "swoop". Can anyone break down what defines this difference? Any additional photos would be greatly appreciated. Thanks in advance!