This is probably the "Trixer 2" upgrade. My "Trixer" had the extra socket (unlabelled) and when I checked, it had all the other features of the Trixer 2. My "Trixer 2" has this socket properly labelled.
It was a factory upgrade, you'd send the unit back to Simmons, they'd do the upgrade and send it back to you, Upgraded software and features...most of which I can't remember at the moment but will try to dig the information out.