The spool action to which I referred was a completely locked carrier without any unlocking action. It was a temporarily malfunctioning locker that unfudged itself randomly. But Jason is dead on with the types of differentials. Easily said: always locked with no hope of unlocking. Some blokes Lincoln lock (welded spider gears to emulate a mini spool) the hp30 and use a posilock (cable actuator to lock/unlock the collar to the two piece passenger shaft and does not affect the differential) to make a poor mans selectable locker.
Luck Favours The Prepared
"Whiskey is for drinking, Water is for fighting over" - Mark Twain