Window spring

Thanks for your photos. Based on what I see here, you have either our Series 380 or Series 385, depending on the depth of the bottom shoe. That's the only difference between them. I recommend measuring your shoe and referring to our dimensions to confirm the best match.

Comparison of the S380 and S385 balance shoes.

To determine the length of the balance, you need to physically measure it with a tape measure. That's the only way to be 100% sure. 

It's important to understand that stamps are not directly representative of the length of the balance, or any other measurement for that matter. They are abstract indicators of how strong the spring is. That said, a common rule of thumb is that the first two digits of the stamp are one less than the length. So in your case, the three balance sizes you'd need should be 29", 30", and 22". 

But you don't want to guess. I still recommend that you measure to verify.

You're welcome! Please let me know if you had any other questions and I would be happy to assist you.