Balance rod stamped 35 9 BSI DX

A purchasing agent from Orlando, FL says:
I already purchased (2) S390-36, option 3510, but this won't hold the window up. I thought the 9 was the weight of the balance. I recently found out that the sash might weigh 40-50 lbs.
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Tom from SWISCO responded:

Sorry to hear you're having trouble. We should still be able to help, but with a window of that size you're looking at something that potentially has to be custom made. Before we go any further, can you give me the exact weight of the sash, down to the ounce? That will allow us to determine what you need beyond any shadow of a doubt.

A purchasing agent from Orlando, FL says:
I went ahead and purchased the 3540 today.
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Tom from SWISCO responded:

Well, please let us know how it works out. If you run into any issues with this size, as well, then we would be happy to walk you through a solution.

A purchasing agent from Orlando, FL says:
thinking someone try to install the balance rod I inquire about, by mistake. I don't know, should be good, the sash is maybe 40-50 pounds. Thanks.
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Tom from SWISCO responded:

Just keep in mind that two balances with the 3540 stamp can only hold 23 through 30 pounds, at max. If your sash really is 40 to 50 pounds then these will be too weak. I highly recommend getting the exact sash weight so that you don't have to guess. It will make your life a lot easier in the long run.

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