My 67 seat back measures 44.5" wide.
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Just noticed on your seat back that the stitch lines don't line up with the seat bottom on the driver's side, that and the fact that the dust cover rail is not on it and I assume no holes thru the vinyl where it mounts, leads me to believe that it is not a convertible seat back.
There may be a metal tag on the seat frame with a part number.
If you remove the seat back take some photos of the back of it and post.
Also if the seat back got bent to more of a curve then it is supposed to have it might produce the same effect you see.
