sash balance

This looks like either an S380-34 or S385-34. 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. Your stamp code of 3390 is from an old rating system we no longer use. To know what stamp option to pick, obtain the exact weight of the window sash. You can then match that exact weight with one of the weight ranges listed alongside each stamp option on the shop page.