I was wondering about that extra nipple on the transmission frame
myself until I encountered an Artecho unit last summer.  The Artecho
uses the same transmission frame and utilizes both nipples.  The large
one, which is opened in the reroll position, works the same as it does
in Ampicos.  The smaller nipple, which is open in the play position,
has a line to a cut-out unit located between the slides on the air
motor.

The way the air motor is set up is quite different than an Ampico.
It is the smallest air motor I have ever seen in a player piano.  The
tempo slide valve is built on the motor, with the governor located near
the pump instead of in the drawer.

The purpose of this cut-out is to by-pass the tempo valve during
re-roll.  During play mode the pouch is open and cuts off the direct
air flow, thus diverting it through the tempo valve.

Brian Thornton - Short Mountain Music Works
Woodbury, Tennessee
http://www.shortmountainmusic.com/