This part is not available as far as I know.  I�ve come across this problem quite often and my solution has been to clean the area well and seal it with some sealing compound: JB weld, Epoxy, RTV Silicone etc.  All that plug is doing is covering an alternate hole for the dip stick tube when this tranny was used in another VW or Audi vehicle.

On May 7, 2014, at 12:00 AM, Automatic digest processor <LISTSERV@GERRY.VANAGON.COM> wrote:

> I have a spare automatic transmission from an '88 Wolfsburg Vanagon that
> works fine, but it has a welch plug (what I would call it) in the forward
> left hand side of the case that has pin holes in it, thereby leaking fluid
> out. The plug looks very much like a "freeze plug"; a convex domed disc
> with a circular sealing rim, only smaller, approximately 23.09 mm diameter,
> installed vertically from the top. Some time ago, Dennis Haynes posted
> about automatic transmission rebuild kits available at:
> http://www.makcotransmissionparts.com/sfnt.html?gclid=CLqb4InN9LMCFQeynQodBB
> but they do not appear to illustrate or list this particular plug. Does
> anyone have any experience replacing this plug, or have suggestions on
> where to obtain one?
> 
> I haven't approached the dealer (yet) about this one.

Cheers,    

Frank Condelli
Almonte, Ontario, Canada
'87 VW Westy, '00 Kawasaki 250 Sherpa, "98 Ducati 750 Monster & Lionel Trains (Collection for sale)
Frank Condelli & Associates - Vanagon/Vanagon Westfalia Service in the Ottawa Valley
BusFusion a VW Camper camping event, Almonte, ON, June 05 ~ 08, 2014