Replacement coil spring

Thanks for posting. This is an interesting setup. That pivot shoe is the kind typically seen with spiral balances--it might be our 15-046. The Coil Balances are our Series 150. Is your old spring stamped '1040'? If so, that likely means it is meant to support 10 pounds individually; we don't carry the Series 150 with that calibration, but you can instead use (2) of the S150-5 5 pound springs on each side of the sash. This will equal 10 pounds of support on either side of the sash, for a total of 20 pounds of support overall. If you want to be extra safe this is right, please weigh the window sash these balances support, and I can work it out for you. 

For a new pivot shoe, compare your old shoe's width and thickness of our 15-140, as seen below. Let me know if anything differs.

Alternate image for 15-140

Finally, for new supports for the springs, use the 15-143, 15-144, and 15-145 for each spring. These 'sandwich' around each spring. Let me know if you have further questions. 

Yep, that's exactly right. You would attach both springs to the pivot shoe, and then stretch one up and over the first so that it rests on top of it. It may be a little awkward to install that way, but there's no other way around it unfortunately. It just takes a little patience and a steady hand, but don't be surprised if it slips out of your grip a few times. It's bound to happen.