OK, I found out something on my '62 GMC Burban ... not sure if this has been addressed or not.
My Burban uses the bolt in style into flanges, (not rails)
Although, the holes are different as compared to the 66 Chevy Burb seats I have ....
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The following pictures are from the REAR (LARGE) SEAT FRAMES for comparison....
NOTE: The older fawn/rusty color is from my 62 GMC
, and the lighter color is from the 66 Chevy:
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As you can see, the lower portion of seat frame is constructed different, and the mounting points in different spots....
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