Is this Milgard balance your 350 model?

I don't think you have our Series 350 balance. It looks like both your bottom and top nylon fitting have wings, while the Series 350 only has wings on the bottom fitting.

I believe you have either our S380-20 or S385-20. The only difference between them is the depth of the bottom fitting. You'll have to measure yours to confirm the best replacement. Otherwise I'm very confident it's one of these two.