Window channel balance

Thanks for writing in. It appears you have the Series 751 channel balance with a knife lock shoe. Since you only have a 20 stamped on the balance, to determine exactly which balance you need you'll have to measure the length of your metal channel. 

Once you have confirmed the length from your balance you'll need to determine which stamp is needed for your window. There is a chart right below the product description for your balance length. You'll need to select the stamp your balance needs based on the weight ranges for each stamp on the chart.

The clip looks like it could be the 19-030 Long S-Clip.

You'll find the balance lengths listed in the link above.