window balancer

Hi Laura. This appears to be a spiral balance, but I need to confirm a few details. I'm afraid that stamp doesn't provide the identification. Can you show us the tip of this balance? Can you also confirm the length and diameter of the metal tube? Do not include the tip in the length measurement. We'll do our best to narrow this down from there. 

Thanks Laura. Based on those dimensions, you have our S900-29. You'll then need to choose a color tip option, which designates the strength of the balance. Admittedly I can't make out the color of the bearing in your photos. The best thing to do might be to weigh the window sash these spiral balances support. You can then match that exact weight with one of the weight ranges listed alongside each color tip option under the product description on the S900-29 product page. Let me know if you have further questions.