New unit needed. The string broke on one side of the window

Thanks for posting. I think you'll need either our S380-27 balance or our S385-27 balance. The difference between the Series 380 and Series 385 balances lies in the shoe. The series 385 balance has the 15-004 shoe which is slightly larger from front to back, as illustrated. 

Comparison of the S380 and S385 balance shoes.

If you measure your shoe as it is pictured here you should be able to determine which one you need. Once you have determined your balance series, you'll want to select spring strength option 2640.