Window balance, no markings

Thanks for posting. This looks like it might be either an S380-27 or S385-27. The only difference between these two channel balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement. See the diagram below for reference. Compare both with your old hardware to see what matches best.

Comparison of the S380 and S385 balance shoes.

You then have to choose a stamp option, which designates the strength of the balance. This is typically identified by a stamp on the metal channel. If there is no stamp, then weigh the window sash the balances support. You can then match that exact weight with one of the weight ranges listed alongside each stamp option under the product description of either the S380-27 or S385-27 shop page.

Let me know if that all makes sense, or if you have any other questions!