Hi John,  I have taken a look inside my 'American' 1/2 Duo-Art and
can confirm that the rearmost nipple has a tube which tees into
a nipple located on the bottom of the cut off block.  This is shown
in a diagram in the Duo-Art service manual which I think I obtained
from John Tuttle's Player-Care.com web site.

The cut off block is to the left of the Duo Art transfer block/switch.
I hope that helps.

Regards,
Andy Hood