Mark, only the SD-709 was used on the 87-91 Vanagons.  I sell these new for
the same price as the Depot if you wanted to order from me.  The SD-510 was
only used in part of '86 (early 86 used SD-508 while later used SD-510 and
then '87 and on used SD-709).  Hope this helps.

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