Channel Balance ID Help - Missing Balance

A home owner from USA says:
I've attached a photo below and it appears that the channel balances were torn out for some reason. The channel width is approx. 5/8" so I believe a 1/2" channel balance would fit, I'm just not sure of the length or the type of end fittings. Judging from the broken plastic fittings at the top and bottom, it looks like this may be a series 350, 351, or 390 (although I am not certain). Any guidance you can provide is much appreciated.

The window is 19.6 pounds. The channel width for the sash balance is 5/8" wide and the visible glass height is 18.5". Can you advise?

Thumbnail of the uploaded file named image.png
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Bob from SWISCO responded:

Is there a balance from an identical window in the house you could show the top and bottom fittings from? Or at least from a similar window? That would help a ton. Can you also confirm the visible glass height? We'll do our best to help from there. Many thanks. 

A home owner from USA says:
Hi Bob,

Unfortunately this is the only window like this in the house. The visible glass height is 18.5".
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Bob from SWISCO responded:

If there is no intact balance to go off of, it's going to be near impossible to narrow this down any further I'm afraid. You said that there were still broken end fittings you were going off of--was this from the original broken balance? If so, we can at least see that? The other issue here is that the visible glass height would imply your balance has a length that does not end in an even increment--in other words, it might be 19-1/2" or 19-5/8", something like that. This is not typical of a standard issue balance, and indicates to me you might have something we can only replace via custom order. Seeing an original balance is the only way to confidently narrow down what you might need. Apologies for any inconvenience. 

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