Transmitters for the Endgate & Hoist (or Endgate only) systems need to be purchased by radio type. 
 
If you have a remote with an antenna on it, you will need to note the color of the band on it (blue or yellow).  
You will also need to identify the color of the LED light in the middle of the display and color of the label.
Red light and green label will indicate it as an S6
Blue light and a red label will indicate it as an S7
 
If you no longer have your remote, you can look at the antenna on the linear actuator and find a colored band at the tip, this will indicate the remote style you need.