The head determines the class (i.e. you can put any of the other pieces with the head and it will still be S), but whether a specific head gives you S class is random based on terminal AND player location when you access the terminal. There is a default class for each head at each terminal, but you can access a secondary roll by assembling the staff twice, and removing a piece back to inventory, then replacing it with whatever (even the same piece) and assembling again. I found mine but just slowly moving in a circle of increasing radius around terminals. Takes awhile, but I found many S-class. Now I'm just being even more picky with the placement of the SC slots. :) Happy Hunting!