Window: side-load single hung balance spring spring need help to identify parts

The plastic attachments come with the balance. In fact, they actually determine the Series of the balance.

Your top and bottom pieces have wings, which matches the Series 385 or Series 380. You have to measure the bottom shoe to tell the difference, since that's the only variation between the two Series.

If you measure as shown below, you can figure out which shoe you need. Ignore the product codes; you won't need to purchase the attachments by themselves. Just focus on measuring the same dimension shown in the picture. The 1" shoe belongs to Series 380, and the 1-3/32" shoe belongs to Series 385.

Then measure the metal bar to verify that its length is 21", as I suspect it is. That would mean you need an S385-21 or an S380-21. The last step is to verify your stamp option.

The stamp is actually printed on your balance, but your manufacturer used a slightly different code system than we do. The 20 4 is equivalent to our 2040. If you weigh your window sash, you can confirm that. 2040 holds 15 to 23 pounds.