The most current policy on the 5 year rule is DoD 1400.25-M,"DoD Civilian Personnel Manual".  

The following link is a memorandum of 12 Mar 1997,  in regards to the 5 year rule: 

http://www.cpms.osd.mil/ASSETS/F224...01_sc4.pdf

I have found out that many commands in Yokosuka fall under COMPACFLT and that COMPACFLT has delegated authority to the Base Commander (CFAY) for approval/disapproval of any extension over 5 years.  Now if CFAY has delegated that authority down to the respective commands, I don't know.  There are many commands in Yokosuka that do not fall under COMPACFLT and their ISIC/TYCOM has not delegated that authority to approve/disapprove to extend beyond 5 years.  So the civilians that have been here for numerous years probably work for the commands that have the authority to approve their extensions locally.

  Also in the memo, it states that if an extension of the foreign limitation beyond 5 years is approved, return rights shall be forfeited. 

There are certain positions held that are exempt from the 5 year rule.  It's in the memo.

There is a change to the CPM coming.  Don't know when, but it's coming. See below!  

Chapter 1200	OVERSEAS EMPLOYMENT
	Subchapter 1230	Civilian Employees in Overseas Areas	To Be Published