On my 1966
tradewind you remove both knobs on the cranks. There is a hole in the side of the crank that you push a small screwdriver into. This push's down a ball on the shaft that is holding the crank handle. Then while pushing the ball in, you pull the crank handle downwards.
Now the screen will be able to be pulled downwards out of the vent frame.
Now you will have access to the mechanism on each side that lifts and lowers the vent cover. I will look for my pictures to see if I can add those for you.