Help with model number

This appears to be either an S380-27 or S385-27 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. You'll then have to choose a stamp option. This designates the weight range the balance can support. Look for a stamp on your old balance's metal channel for reference. If there is none or you're unsure, weigh your window sash. You can then match that exact weight with one of the weight ranges listed alongside each stamp option on the balance shop pages. 

Comparison of the S378, S380 and S385 balance shoes.