My '80 Westi has/had the piece of metal you all are referring to.  It 
is a heat shield, mine just bit the dust a few months ago.  I will 
check my engine seal. I still have the bulk of it remaining.  <What 
sort of packrat would keep a burned up, unattachable former piece of 
VW> A VW owner, that's who.
It's not a sandwich though, just a piece of metal.

> >original equipment beyond the heater boxes. Was that the purpose of this
> >piece and if so, would it aid cooling to rig a shield over the muffler?
> 
> I think it's a heat shield.  It's a very hard to find piece, because they
> tended to rust out and fall off.  I ran without one for a while and it
> eventually caused my foam engine seal to get crispy on the lower parts in
> the back.  I'm sure it affected the temperature of the air entering the
> engine, too.  Good thing to have, if you can get it.

I just replaced the plastic generator cage, about $7.50 US
> a recent problem saw the end to my fan cage and generator surround. 
- Jeff '80 Westi