We have three versions, which do not exclude each other:
A. The center back seam (common in tailoring) incorporates subtle fitting steps and helps make alterations at a later stage - I looked at Claire Shaeffer's Chanel collection in her Shaeffer on Chanel CD, and yes, there are quite a few jackets with center back seam. It is probably the era - I prefer shaping with princess seams, but
B. Some of the fabrics for Chanel designs are woven on narrow looms, so it would have been impossible to construct the jacket without a center back seam
C: The Hem: After studying these images again and again, I think the jacket was shortened at some point. This would explain the unusually bulky hem and the transition from quilting to the hem at the bottom of the jacket.
Lets look at the pictures again. The quilting lines look like they were not restitched, so the center back seam is part of the original design that's for sure.
Now, look at this hem. It looks like there are more layers there, and that it was hemmed by simply turning over the quilted portion - it looks like a double hem. I don't remember seeing anything like that before - usually the lining would be attached to conceal the hem edge. Is it a bad alteration job?
Look at the hem portion again - it does look pretty bulky or.
What do you think about the hem, readers? Was it altered?