Hello there. I see you had the old Simonton pivot shoe that is no longer made. The 15-259 should be able tp replace this; however you also need to replace the coil springs with Series 159 springs. The coil balances that worked with your old shoe are not compatible with the 15-259. You have to replace the entire setup for this substitute to work. If you're uncertain of what spring weights you need for the new Series 159 springs, give me the exact weight of the window sash, and I can work that out for you. Also see the video below.
Hey Charlie. If the sash weighs 21.2 pounds, then you can just use (2) of the S159-10 10.5 pound springs, one for each side of the sash. That will equal 21 pounds of overall support, which is more than close enough to your reported sash weight. The actual number of coils on either side of the sash doesn't matter as much as hitting that weight range, and since you're using a new Series the number of coils needed to reach that range is different. Because of this you can use the 15-255 Single Supports for these coils. Let me know if you have any other questions, and thanks for the kind words!
