Take a look at the S395-18 balance. Are the attachments the same ones that you have on your old balance? Is the metal channel 18" long? If so, this is your balance. You'll need to weigh the window sash to get a stamp "option." It might be "1720," a variation of the "17 2" on your old balance, but the only way to be certain is to weigh the window sash and look for a chart on the product webpage.
