I believe the 5.5 and stands for Frequency.  Per the KBS web site Reg is a frequency of 5.1 and Stiff is 6.1.  I hope this helps

I spoke with our shaft R & D to see if the ProLaunch Red hybrid was a shaft we were testing or planning to add to our matrix and this is what they said:

Grafalloy chose to use a different basic structure for the PL Red Hybrid than the PL Red iron…

Thanks Nick!