I don't think the silver secret was installed by the dealers.  I'm the
original owner of the 82 Westy, and the dealer would have been VERY CAREFUL
to charge me for any add-ons!  This camper came out of the North Central
region, and my sister's '80 silver-secret-bearer came out of whichever
region serves southern California.

I've never seen any electrical adapter, in the auto or electronic world,
that matches this dang socket.

I'm wondering if it had something to do with an auxiliary battery?  Some
of the service manuals talk about an auxiliary battery to be located under
the DRIVER's seat - a mirror image of the 'normal' location, but I've 
never seen an early Vanagon US Westphalia with the second battery.  Maybe
the silver secret was meant as an INPUT, for charging, rather than an
output?

P.S. - as a newbie to this list, please chalk me up as an 82 Westphalia,
located in WI.