CPR overflows are not exactly classified as J tube tyles but more or less a C shell style. This is because the chamber that water passes up and over the lip of the tank runs the entire lenght of the overflow vs a U tube or J tube style that has independant 1" tubes.
Water enters one side of the oveflow (the side in the tank) and via syphon action goes up and over the tank side to a second chamber. This chamber has a wall that is often only 1/2 the height of the chamber's other 3 walls. Water then cascades over this shorter wall into another chamber that has a drain fitting that allows the water to drain into the sump.
Since water will naturally try to equilize its level the outside chamber is usually little lower in water level than the main tank. If the U tube remains free of air water will flow up and over the side to try to rase the outside chambers water level but since the shorter wall never allows the water levels to equlize then the overflow runs consistantly untail the pump putting water into the tank stops.
I would check out the DIY
forum here for info on how to build your own overflow. You can also check out http://www.lifereef.com
for quality U tube overflows.