Yes, they used welting. This is an original 1950, 3604 seat
before it was replace.
http://www.pbase.com/dennygraham/image/120777354
Here's a close up of the welting.
http://www.pbase.com/dennygraham/image/161399365
My bet would be that the '53 was still using welting in the
seams.
But these are pictures of an original '54 seat, they are
calling it a French seam, but it's really not a french seam
in the true sense.
http://www.pbase.com/dennygraham/upholstery

Denny G
Sandwich, IL