old balance spring broke, can't find the replacement

Hello there!

Thanks for reaching out to us. What you have here is an old rod-style balance. They're not made anymore but one of our modern balances can replace them. I'm just not sure which one you'll need from this photo alone. There should be a type of shoe that goes with this balance, perhaps still installed in the window track. May we see photos of the shoe that goes with your balance?