This cap is secured to the window and seems to be the strongpoint that the balance pushes against. Plastic mold.
Measurements:
-The last pic shows the top of the cap. You'll see that there is a mold line across the top of that surface. That line measures 1-1/4". The cap measures 1" in the other direction.
-The insert portion of the cap seems to be 9/16" W x 7/8" D x 5/8" H.