single hung window channel balance

A quick learner from Arizona says:
I need to replace a broken channel balance but am not entirely sure which is the correct replacement part. Thank you for your help!
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Tom from SWISCO responded:

I'm not sure what you have, unfortunately. We have dozens of different balances with that stamp. It's not a product code or anything, it just indicates the strength of the spring.

In order to figure out what Series this is, I need you to remove an undamaged version of this balance from the window and show me clear pictures of the top and bottom nylon fittings. I will also need to know the dimensions of these fittings.

A quick learner from Arizona says:
So after looking at the balance itself, that isn't what is broken. I am missing the top piece that holds it inside the window track. Here are some pictures of the top and bottom of the channel.
Thanks!
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Tom from SWISCO responded:

Thanks for your photos. Based on what I see here, you have either our S351-21 or S390-21, depending on the depth of the bottom shoe. That's the only difference between them. I recommend measuring your shoe and referring to our dimensions to confirm the best match. You will need an undamaged version of this balance to use as a reference.

As for the piece that holds it in place, it sounds like you're referring to the top sash guide. We should be bale to help with that, as well, but I need to see one removed and in undamaged condition. I specifically need to see it from the front and side (like a mug shot). 

Comparison of the S351 and S390 balance shoes.
Comparison of the S351 and S390 balance shoes.

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