Same wedge shoe, slightly different mechanism

A landlord from Massachusetts says:

My existing balance does not have the steel channel around the spring and bushing rod.

I'm fairly certain the 775 is the right part to replace it but I can't quite determine how to measure for a compatible fit.

Is there another way to determine the proper size? Height of sash minus a few inches? Height of entire window divided by 2? Length of fiberglass rod plus one?

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Bob from SWISCO responded:

Thanks for writing in. There's a few possible replacements for these old arrow balances, with the Series 775 being one of them. But I can't say for sure which channel balance is the right replacement without seeing the end attachments of this old balance; for instance, the wedge-shaped shoe of the Series 775 . Show us the end attachments of your old balances, and we should be able to narrow down which is the right replacement. Many thanks. 

A landlord from Massachusetts says:
Here's the shoe. It's 1-1/4. How can I measure for the replacement based on my broken assembly? I only have some of the parts. They were taken out and replaced with sticks in the past. I was hoping I could add a number to the length of the rod or the size of the sash.
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Bob from SWISCO responded:

Thanks for getting back to me. This definitely looks like the shoe to our Series 775 , so that's the right replacement option for you. As far as estimating length, is there any way you can put one of your old balances back together well enough to measure out the size you need? If not, I'm forwarding this discussion to my colleague Paul just in case; he has more experience with these balances than I do, and may have an alternative solution. 

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Paul from SWISCO responded:

Perhaps I can be of assistance. The rod you are holding in your first photo: could you give us the length from the top to the very end of the tube? I should be able to help calculate what you need from there. Many thanks. 

How to install a S775 balance
How to install a S775 balance
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