What replacement part should I buy?

This is either an S380-30 or S385-30 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 need to choose a stamp option. Your stamp is a little unusual, so I advise obtaining the exact weight of the window sash. You can then match that exact weight with one of the weight ranges listed alongisde each stamp on the shop page. 

Comparison of the S378, S380 and S385 balance shoes.