Channel balancer

Thanks for posting. Is the metal channel, not including the nylon end attachments, 15" long? That's what it looks from the photos, but photos of measuring tape can be deceiving, so please confirm. If so, I think this is either an S378-15, S380-15 or S385-15 Channel Balance. The only difference between these balances is the 'depth' of the bottom nylon shoe. See below. Measure your old balance's shoe depth to see what matches best. 

Comparison of the S378, S380 and S385 balance shoes.

You also have to choose a stamp option, which reflects the balance's strength. Look for a stamp on the old balance that indicates this. If there is none or you are unsure, weigh the window sash these balances support. You can use that exact weight to choose the correct stamp option.