How to order correct replacement for this window balance

A handy person from St.Paul MN says:
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!
User submitted photos of a window balance.
Profile picture of Tom
Tom from SWISCO responded:
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.
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