I'm sorry, but I don't recognize this Series coil balance. I will need some additional information before we can research a replacement.
First, please remove the window sash and take a photo of the pivot lock shoe. In your pictures above, the sash is obscuring important details. I will also need to know the width and thickness of the pivot shoe. These dimensions are essential! Please also take a photo of the bottom pivot bar on the sash. Finally, how is the spring attached to the shoe, and what is the width of the spring?