Need to purchase correct channel balances
Hello there. This looks like either a Series 380 or Series 385 Channel Balance. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. See the diagram below for reference.
Next, you'll need to confirm the balance length. Be sure not to include the nylon fittings at both ends of the metal channel in this measurement. Finally, you'll need to choose a stamp option, which denotes the strength of the balance. Look for a stamp somewhere on the metal channel that indicates this. If there is none, or you're uncertain, just weigh the window sash the balances support. You can then match that exact weight with one of the weight ranges listed alongside each stamp option on the balance shop pages.