1/2" channel, 21"long has marking 20 9 CALDWELL on it.

Hello there. See the S380-21 or S385-21 Channel Balance. Depending on the shoe depth, one of these should be what you need. See below. 

Comparison of the S378, S380 and S385 balance shoes.

You then have to choose a stamp option. This reflects the balance's strength. Your old stamp of 20-9 is from a rating system no longer in use. To know what stamp to select, weigh the window sash these balances support. You can then use that exact weight to choose the correct stamp option.