My Nock Centering Tool (NCT) is again specifically design for Ravin crossbows since they have a railless design.  For any new Ravin owner who has experienced how difficult it can be to nock and arrow on a new Ravin string, this tool was designed for you.  It will not only make it easier to nock and arrow on the string but prevent lateral nock travel at arrow launch.  
It also can be useful on double served aftermarket strings.  I personally always remove the center serving on a new Ravin string and double serve it before installation.  I then use the NCT to put a concentric nock notch in the new serving, but only after the string and cables have settled in a bit.