Unusual repair

[Ray P]

I am starting a ground up restoration for my 30 coupe. After having the body media blasted I was surprised at all the damage that was Suddenly visible. There was the usual rust through, cracked fenders and miscellaneous dings etc. the big surprise though was a weld seam on both quarter panels just above the belt line that runs from the door opening all the way around to the rear roof panel. The apparent weld is highly visible from the inside and somewhat visible on the outside. Any ideas what this may be? I'm only able to attach one pic