Window spring help

Hey Kyle. I can't quite make out the bottom nylon fitting, or the 'shoe', in these photos. Does it happen to look like either of the shoes in the diagram below? If so, I believe you have either the S380-33 or S385-33. The only difference between these two channel balances is the depth of that bottom shoe, or what you can consider the 'heel-to-toe' measurement. Again see the diagram below for reference. Compare both options with your old balance's shoe to see what matches best. In either case, for a new stamp option choose 3250.

Comparison of the S378, S380 and S385 balance shoes.

That's a very common issue. Odds are your shoe was once the larger 1-3/32", but the plastic wore down from friction over the years. It happens all the time. I feel fairly confident that the larger Series 385 is what you're looking for.

You're welcome, Kyle. Please feel free to reach out if you needed anything else.