Side Channel Non-Flipout window

Hello there. This looks like either an S380-33 or S385-33 Channel Balance. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. To measure this, you'll need to produce a balance with an undamaged nylon shoe, perhaps the other from this window or from an identical window elsewhere in the house. The image below depicts the depth difference. 

To know what stamp option to select, obtain the exact weight of the window sash. You can then match that exact weight with one of the weight ranges listed alongside each stamp option on the shop page. 

Comparison of the S378, S380 and S385 balance shoes.