Hi Tim and MMDers:  The two smaller central pneumatics are held in
the open position at atmospheric pressure with springs at the rear.
They hold the tracking system in a centralized position.

During play vacuum is applied and these pneumatics retract out of
the way allowing the main pneumatics to keep the roll aligned.
During rewind the small central pneumatics open up and centralize
the system if play ended with the top spool off centre.

Tim, let me know if this is not clear and I can send you photos.

Regards
John Johns
Ottawa, Canada