Need help to determine correct replacement part.

Thanks for posting. This is called a channel balance. Its the spring-loaded mechanism that helps regulate the opening and closing of your window. I'm confident we can replace this, but I'd like to be sure we know exactly what type, or Series, of channel balance you're working with. That stamp actually just tells us the strength of your balance, not its identity. To know the identity, can you show us clearer pictures of the nylon fittings on the top and bottom of the metal channel? Their exact design will clue us in as to exactly what you need. Many thanks. 

That's perfect, thank you. See our S395-31. For a new stamp option, choose 3030. Let me know if you have further questions.