Window springs single pane

Thanks for posting. At first glance, it looks like you have either our Series 380 or Series 385 Channel Balances. The only difference between these two balance Series is the depth of the bottom nylon shoe, or its measurement from 'heel to toe'. See the diagram below for reference. Measure the depth of your old balance's shoe to determine which you need. I think it is more likely you have the Series 380, which has a 1" shoe depth; however, it is also possible your shoe's depth is less than that. Let me know if that is the case, and exactly what the measurement is if so. 

Comparison of the S380 and S385 balance shoes.

To know what length of balance to select, measure the metal channel. Do not include the end fittings in that measurement. Finally, you have to choose a stamp option, which denotes the balance's strength. Look for a stamp on the metal channel that indicates this. Let me know if you have any further questions or issues.

If the shoe is 1" deep, then you need a Series 380. I believe the metal channel, not including the attachments on both ends, will be 29". Confirm this. If so, use the S389-29 S380-29. Choose 2830 for a stamp option, which accords with your reported old stamp of 28C. Let me know if you have further questions.