Channel Balance ID

A home owner from Kansas City says:
I have 4 separate windows that use these. The numbers on the backs are 3640, 3620, 2440 and they are missing from one large picture window shown above completely. The weight of the window sash is 24lbs. I'm not sure what series they are and several series you have appear very similar. Please advise the best choice. Thank you
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Bob from SWISCO responded:

Thanks for writing in. The balance you show in your photos looks like either a 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. Make sure all 3 sets of balances have the same fittings. Be sure to measure the length of the metal channels, not including the nylon fittings, to know what lengths you need to select. The balances with the stamp options of 3640 and 3620 should be 37" long, while the balance with the stamp option 2440 should be 25" long. Let me know if you have any other questions. 

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