Which replacement parts?

A home owner from Brooklyn says:
The brand name is Repco. We are looking to replace channel balances on four windows that are likely from the 1980s. The number on the metal part reads 29 C and 3.S.1, they are 30" long.

I can't figure out if we need this:
https://www.swisco.com/30-inch/pd/Series-351-Channel-Balances/ S351-30

or this:
https://www.swisco.com/30-inch/pd/Series-390-Channel-Balances/ S390-30

We probably also need these:
https://www.swisco.com/Take-Out-Clip/pd/Channel-Balance-Retainers/ 16-002

What seems right?

Thank you!

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Bob from SWISCO responded:

I agree you have either an S351-30 or S390-30 . The difference between these two balances is the depth of the bottom nylon shoe, or 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. See the image below for reference. For a stamp option choose 2930. 

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