Directly from Mavic
SUP
Concept :
After bending, the rim joint is arc welded.
The welded seam is then milled by digitally controlled machines.
Benefits :
- On a SUP rim, the resistance of the joint can increase to 90 %. On ordinary sleeve-joint rims, resistance at the joint is
40 % to 60 % lower than on the rest of the rim
- No visible joints that could cause shuddering during braking.
- The "monobloc" geometry makes wheel building and balancing easie.
http://www.mavic.com/servlet/srt/mav...nosid=18&lg=uk

Not just "current" rims but older rims as well. Such as the venerable 217 SUP and the 117 SUP and the 117 SUP CD all of which are many years old.