when a frame is manufactured with the resistance welding process they put opposite electrical charges on frame components and jam it together with force when an arc is established, like a giant spot weld. There is on occasion a failure of a joint to properly weld. In those cases, that frame could be run through a fillet brazing process to get it back into the production loop. That would be my guess. Or while the process was being developed they only ERW'd sub assemblies.