existing lower (29") & upper (27") sash spiral balance hardware - to be replaced

A quick learner from Bedford, NY says:

Could not find any manufacturer's name, #s, or marks on any of the balance system

hardware.

I have not gotten an opportunity to weigh any sash yet, but I believe this room's lower sash weigh about 20lbs.

the existing spiral hardware is currently attached to the bottom of both sash by the 3" X 3/8" pivoting plate @ the bottom of both sash's spiral balance hardware (shown in the photograph. the existing spiral balance system appears to have replaced the original sash weight balance system - quite a few decades ago, I would venture to say.

We are intending to replace the balance system hardware in about about 20 sash openings in this building

Thumbnail of the uploaded file named half of both upper and lower pair of spiral balanc
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Paul from SWISCO responded:

Thanks for writing in. I think it's safe to assume your spiral balance is 5/8" and are made for a wood window. If this is correct, then you should be able to use our S500-27 and S500-29 with a 17-007 balance bracket. Compare your parts to ours just to make sure this is correct. 

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