[Mark IV]

Plug Weld - Also called a "rosette weld", two pieces of tubing are welded together, one inside the other, through holes drilled in the outer sheath.

Opening the end of the frame tubes and inserting another tube has been a semi-common Cobra restoration/frame reinforcement/strengthening technique for some time. The original outer rails stay as they were and when you grind/finish the Rosette welds it is un-noticeable.