Dual arm operator

Looks like you have either our 39-253LB or 39-438B. The only difference between these two operators is the appendage at the end of the straight arm. See the video below for a clearer explanation. 

The third arm that you see is a hinge like our 36-048 hinge set. 

I agree with Bob's recommendation that you need one of our Encore operators shown in the video he sent you but you'll need to detach the long arm from under the window sash to determine which attachment it uses compared to the video. Thank you!