UNIQUE 17-B-20

I suspect that if you measured the metal bar from end to end, it would be 18". Is that so? If it is, you can use the S350-18. You may need the 1720 option, which holds between 8 and 12 pounds, but the only way to be sure is to remove the window sash, weigh it, and compare to the chart in the product description on the S350-18 webpage.