IIRC the CXP-33 used an internal nipple reinforcement/ferrule. As the nipple is tightened this reinforcement device is pressed against the rim and trapped in place. Almost none of it can be seen from the outside so the rim looks clean and unreinforced. The OEM version of this rim (CX-33, ?) is without this device. But this rim is extruded with a thicker wall at it's peak, to better withstand the spoke tension. Of course this makes for a heavier rim and one that costs less. Both qualities that on a new bike are OK to choose (by the bike brand).

Both the CXP-33 and it's lower cost OEM version look almost exactly the same on the outside. So I've dealt with a number of customers who were "fooled' by this. They thought they had the better rim...

The biggest problem with the no reinforcement design is the nipple seating on bare Aluminum and the resulting high friction between them. This makes building and future truing more nipple stripping likely. Using a lube on the nipple seats helps at the time of building and I often add TriFlow to each nipple/rim point prior to truing built and used wheels.

On a side note my wife uses these rims on her Sunday bike. I can't wait to build her next bike and will pick traditional box shaped rims (Mavic Open Pros or Sports, with butted spokes) as well as get her off the overly stiff Alu frame. She'll have a much smoother ride without the vertically stiff crap she rides now. Andy.