I recently started repair work on a Duo-Art expression box.  Once
I removed cloth from the Accompaniment pneumatic and exposed the knife
valve, I saw that the block with the vacuum passage had a second
passage through the top of the block.  There may be a part missing.
I never had seen that before, nor had another re-builder that I know.
We're not beginners.  I hope someone can provide information about this
variation.  I put a picture of the part on the web, at

  http://stevemanngtr.com/dax/pict.html 

Howard Goldberg