How to order correct replacement for this window balance

A handy person from St.Paul MN says:
October 13, 2016
I have a window balance labeled FA3040, based on the measurements I believe it has a FS100 top shoe and the FS151 bottom shoe. It is 31 inches in length. Is the equivalent model number to order from your website the S351-31. Does that come with the shoes attached or do I need to specify somewhere in the order that I want the shoes on it? Thanks!
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Tom from SWISCO responded:
October 18, 2016
Thanks for posting. This could be either our Series 390 or Series 351 but I'm not sure which would be better. It would depend on the depth of your shoe; yours looks pretty large.

Both balances come with the nylon fittings pre-attached and ready to install out of the box. I would just go over the dimensions we show to confirm which is a better match.
31" Series 390 Channel Balance
31" Series 390 Channel Balance
S390-31
$11.30
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