We're the second owners of our Windsport. When we bought the rig, it had a fiberglass screen with velcro attachment; appearing to be a user-addition just as Pete described. We added a "cut to length" piece of 1/2 x 2" wood that makes a perfect brace between the glass frame and window frame to safely prop it open. That comes in really handy for air circulation, and enjoying the night air and sounds in the spring and fall.
I also used a piece of nylon screen cut to fit and attached velcro. I ended up using gorilla glue to get the screen to stay attached to the velcro, the velcro on the window frame was fine. I cut a little access flap at the bottom to unlock and place my little wood prop in the window, then re secure with the velcro attached.