Need a channel balance replacement

Thanks for posting. I am confident we can help, but to be extra safe, can you show me a picture of where the spring and pulley system connect in the 'guts' of the metal channel? There's just one detail with your balance I want to be 100% certain of. Also, can you tell me the exact weight of the window sash that this channel balance supports? We should be able to square everything from there. Many thanks. 

That's perfect, thanks. This looks like either an S380-30 or S385-30. The only difference between these two channel balances is the 'depth' of the bottom nylon shoe. See the illustration below for reference. Measure the depth of your old balance's shoe to determine what meets best. 

Comparison of the S378, S380 and S385 balance shoes.

You then have to choose a stamp option, which represents the strength of the balance. Your old balance's stamp of 29-9 is from an outmoded rating system. For a window sash that weighs exactly 33 pounds, choose 2960, which denotes a 30" balance that can support a sash in the weight range of 32 through 35 pounds. Let me know if you have further questions.