35" x 5/8" spiral balance with a yellow tip?

To my knowledge, no balance in the 5/8" size was ever made with a yellow tip. That's exclusively made for 3/8" balances. Can you verify that this is the size you need? Can you also confirm that your tips are in fact yellow? It's possible that they were once white and simply faded with age.

If you are positive that the balance tubes are 5/8" in diameter then odds are you need a different color. I would recommend weighing your sash on a scale to confirm exactly which available tip is best suited for this job.

Hello! I agree, your tip was definitely white at one time. White Tips from that era are usually the same as modern Red Tip; at some point in recent decades they started using Red to replace all the White. Also, looking at the diameter of your outer tube, I think the larger balance is the safer bet. The 5/8" size is more like 9/16" in reality (I don't know why the manufacturer rounds up), which is just about what yours is. 

So, taking into account the size and diameter, that would be our S600-35. The Red Tip option carries 12 through 30 pounds, which would be perfect for your 20 pound sash. That would be my recommendation.

The Series 600 is compatible with about a million and a half different pivot shoes. Yours in particular looks like it could be a good match for our 15-046, but you will need to first check to ensure that all our dimensions line up. There are plenty of other options, so if these dimensions are no good then just let me know and we can pursue something else.

Alternate image for 15-046