I make the water tank keys all the time.  Key blanks are available (and in
stock).  I need the lock to make it from, however.  There are no code
numbers on the locks, as the code number was only on the original keys, and
there are well over 100 different possibilities for key cuts.  If you need
further information, please let me know and I can give more details.
Steve Sandlin
http://www.steveslockshop.com
'84 Wolfsburg Westy
'79 Transporter
'00 New Beetle