Was able to get the old blueprints and now see that it fed a unitvent panel. I am not sure why, as the panel also had (according to the print) a set of #8 feeders to it.

Cool set of prints!

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The unitvent panel was right under my nose, but has been retrofitted and I had no clue to think it was that close, The pic of the cut out flex in my earlier post aimed right at this wall panel. There was only about a foot of flex to reach the wall, as you can see here.

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Here is the re-fed and retrofitted unitvent cabinet

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