Window balance spring loaded pulley channel 21-12

Thanks for writing in. This looks like either our Series 351 or Series 390. The only difference between these two balances is the 'depth' of the bottom nylon shoe, so compare both with your old hardware to see what matches best. To know what length balance you need, measure the metal channel, not including the nylon fittings. Finally, your stamp code is a little weird to me. Weigh your window sash. You can then match that exact weight with one of the weight ranges listed alongside each stamp option on the product page.