Window slider/spring part?

A handy person from Minneapoolis, MN says:
October 2, 2017
Hello,
We are selling our condo and have a buyer who has asked us to fix a double hung window (I was not aware of an issue). But came in and saw this piece hanging off. I was able to remove it fairly easily.

Looking to find the part and possible insight into how to repair it.

There is an alpha/numerical marking on the solid metal pieceL:

30DS38(written horizontally) BS1(written vertically) PV(written horizontally.

The string on the piece also looks like it is broken, the spring seems to be in fine shape.
Chanel Balance
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Tom from SWISCO responded:
October 6, 2017
Thanks for posting! This is a special kind of balance that will need to be custom made, due to its heavy spring strength. Before we get started, can you just tell me how many you need?
31" Series 390 Channel Balance
31" Series 390 Channel Balance
S390-31
$11.30
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