parts fitting together

If your old balance was stamped 23-10, then I would stick with 2310. If you want to be 100% sure, you can always weigh your sash on a scale and refer to the chart on the S385-24 store page. You don't want a spring that's too strong.

I also cannot confirm if the 18-147 will work for you. The S385-24 is compatible with a TON of different top sash guides. The only way to know if the 18-147 is a match is to compare our dimensions and pictures to what you have installed now.