window balance identification

This appears to be either an S380-18 or S385-18. The only difference between these two balances is the depth of the bottom nylon shoe, so compare both with your old hardware to see what matches best. The picture below illustrates the difference. For the Series 380, the shoe is 1" deep; for the Series 385, it's 1-3/32" deep. 

In either case, you'll need to select a stamp option of 1740.

Comparison of the S380 and S385 balance shoes.

If your balance shoe is over 1", then you want the Series 385. It was probably once 1-3/32" originally and wore down over time.