Missing numbers

Thanks for writing in. To start, it looks like you have either a Series 351 or Series 390 Channel Balance. To know which one you have, compare the 'depth' of the bottom nylon shoe. Next, measure the length of the metal channel not including the nylon fittings to know what length you require. Finally, you need to select a stamp option. If there is none on the metal channel, or if you're uncertain, weigh your window sash. You can then match that exact weight with one of the stamp options listed on the product page.