CHANNEL BALANCERS

The number you listed actually isn't the model number. It's a manufacturer code, but it is useful. It tells us to choose a 20" balance and a 1950 stamp option. You can verify the length by measuring the metal channel (just the metal) and the stamp should match up to the chart on the item webpage, which relates the weight of the sliding window sash to the stamp on the balance. As for your Series, you have either the Series 380 or Series 385. Measure as shown below to determine which. A 1-3/32" shoe, as in the photo, indicates you need an S385-20. If the shoe is only 1", you have an S380-20.