Window balance 26 - 4

Hello, thanks for posting! It looks like you could have either our S380-27 or S385-27 with the 2640 stamp option. The difference between the Series 380 and the Series 385 lies in the depth of the shoe. Take a look at the picture below for more detail. Once you have determined this dimension you should be able to select the appropriate balance. 

Comparison of the S378, S380 and S385 balance shoes.