That ground tab was for a shield that the older waterboxers had to help
lessen radio interferance from the distributor cap.  Apparantly they found
that they didn't need it because all of the newer Vanagons never had this
shield (but the distributors still had the tab).  However on new distributors
this vestigial tab is missing.  The only thing it is good for is a handy
place to ground that annoying oil buzzer wire :)  Don't worry about it.

Ken Wilford
http://www.vanagain.com
John 3:16
Phone: (856)-765-1583
Shop:   (856)-327-0027
Fax:     (856)-327-2242