Once the WBC broke from the NBA (soon to be the WBA) in the early 60's, the ring magazine would only crown a vacant world title when the two titles were unified. Once the IBF came around in the early 80's you had to win all 3. The Ring has never recognized the WBO. Now that they have their own system again they don't care about any of the alphabet bandits and rightfully so.

http://www.cyberboxingzone.com/boxing/pastchp.htm

Cyber Boxing Zone has a pretty fair lineage.