Help on identifying right part

A quick learner from Indiana says:
Hi this is my 18 window balancer which has a stamp so I belive its a 400 series ? But it's the only park on it and I don't know what weight to use to choose right replacement part.
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Bob from SWISCO responded:

All channel balances have stamps that reflect different weight ranges they can support. I am confident we can replace yours, but the only way to know the identity of this balance is to have clear pictures of the nylon fittings at either end of the metal channel. We can do our best to help from there. See the video below.

A quick learner from Indiana says:
Okay iv attached more photos thanks and it's 19 inches long and almost 1/2 wide
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Bob from SWISCO responded:

Thanks for getting back to us. This appears to be either an S351-19 or S390-19 . The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. See the image below for reference. If that stamp reads 18B, then use our stamp of 1820. 

S390vS351
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