Window Tracks Rail or Balance

Thanks for posting. I am confident we can help, but I am afraid we need more information. Those stamps only tell us the strength of the channel balance, but not its identity. To know what kind of balance you have, we need to see the balance out of the window and with clear photos of the nylon fittings or attachments on both ends of the metal channel. The combination of attachments determines the balance's identity. See the videos below for further guidance. 

Assuming that your window is the same exact make and model, only differing in size and sash weight, then we can reasonably assume that the Series 380 channel balance is used for both applications. Based on the stamp you showed in your first photo, you would use the S380-27 with a 2640 stamp option.

That said, you should absolutely make sure that the top and bottom fittings match those that come with our balance before you buy anything. This window being the same make as the other one in your house means that this is a safe bet, but at the end of the day it's still a bet.

No problem at all! I was happy to assist. Feel free to reach out if you had any follow up questions.