Balance

Thanks for posting. This looks like an S351-26 or S390-26 Channel Balance. The only difference between these two 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 S351 and S390 balance shoes.

You then have to choose a stamp option, which designates the strength of the balance. Your old balance's stamp of 2590 is from an outdated rating system. To know what stamp to select, weigh the window sash that these 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 balance's shop page.