Hello Arthur and MMD,  I checked my 1926 Stroud 1/2 Duo-Art that 
I rebuilt a few months back.   On mine, that nipple runs off the back
of the exhauster and up to the junction block from the lower section 
to the upper section.  From the junction block in the upper section
that vacuum supply runs directly to the tracking pneumatic giving full
unregulated power for the tracking mechanism.   

I hope this helps.  If you're interested, when I rebuilt mine I took
the time to make a full tubing schematic which I would be happy to share.

Regards,
Alan Turner