As mentioned on the other forum, I believe it has a different angle where it meets the Rear Quarter Glass, but that would need to be verified by someone who would know for sure.
It also could be that it has a different angle where it meets the Roof.
I do remember being told by more than one through the years that it has a different angle somewhere.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America