Art -- I think you mean a 7/32" elbow (for 3/16" tubing).  This is
intended to provide a constant supply to the automatic tracking device
so it will center during reroll.  There was normally a cutout pouch
block that supplied suction only during play, to disable the tracker
ear valves during reroll, as otherwise the correction would be reversed
with damage to the edges of the roll resulting.

If the cutout block is bypassed or done away with and its supply
connected to the tracking device, the tracking pneumatics would not
center during reroll (as some prefer because of right-hand flange
dimensional variations), the elbow on the back of the reservoir would
no longer serve any purpose, and you would also have an unused
connection point at the top left of the stack.

Jeffrey R. Wood