Trouble figuring out correct spiral balance.

Hello there. One rule with spiral balances is that if you ever encounter one that has a length ending in a X-1/2" increment, it's best to round up. Therefore, you're going to need either our S300-29 or S600-29. Now comes the question of diameter. You cite your spiral balance's diameter at 3/8", but it's missing its outer tube, which was likely a plastic sheathing that eroded away. Our spiral balances use a metal outer tube instead that is more durable. I suspect that your outer tube's diameter was indeed 5/8"--especially since the 5/8" spiral balances are the only ones that use blue tips. Therefore, you would need our S600-29. Let me know if you have further questions.