Balance fell out of my window
Hello there. This looks like it's likely either an S380-32 or S385-32 Channel Balance. The only difference between these two balances is the 'depth' of the bottom nylon shoe, or the 'heel-to-toe' measurement. See below for reference. Measure your old balance's shoe depth to see what matches best.
You then have to choose a stamp option, which reflects the balance's strength. Look for a stamp on the metal channel that designates this. If there is no stamp or you are unsure, weigh the window sash that these balances support. You can then use that exact weight to choose the correct stamp option. Let us know if you have further questions.
Great. That means you need an S385-32, with a stamp option of 3140--the BSI is just manufacturer info you needn't worry about. Let me know if you have further questions.
Good question. Usually, yes--if one fails the other is usually on the outs as well. Plus, mixing a new balance with an old one is not a good idea, since the newer balance will have to do more work to compensate for the older counterpart, which usually results in both breaking more quickly. For those reasons, we always recommend replacing both balances in the window at the same time.