XA is nothing to do with prototypes. My III & IV also XA serials and I have one more with XB and regular B. X probably means they were the ones with the active Korean HAZ preamp. Korean Spectors also had serials beginning with X and they were far from prototypes.
The forum I wasn't with passive features. ALL Forum I & II has ACTIVE EMG with Spector KHAZ preamp. The difference between I & II is the neck through vs. bolt on construction and the I is arched top while the II is flat.
With the Forum III-IV there were not option between passive and active. The ones with