Ha, I had the same thought.
The only Centris that was different from the Quadra of the same name would be the 610. The Centris ran a 20MHz 'LC040, whereas the Quadra had a 25MHz '040 - their motherboards were very different. Of course, everything else (drives, cables, casing, PSU, etc) was interchangeable.
The Centris/Quadra 650 and 660av were virtually identical, however, save for some different configurations of drives or memory.