21 9 BX CMC 11

Hello there. This looks like either an S380-22 or S385-22 Channel Balance. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement. Compare both with your old hardware to see what matches best. See the illustration below for reference.

Comparison of the S378, S380 and S385 balance shoes.

You then have to choose a stamp option, which designates the balance's strength. Your old balance's stamp of 21-9 is from a rating system no longer in use. To determine the correct replacement stamp, 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 the S380-22 or S385-22 product page.