You can access it over SSH via USBNet, which *should* only require a working JB to install

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I don't recall if anyone maintained any such recovery image/dumps, but I'd imagine the hack that modified the file either made a backup somewhere, shipped with an uninstaller, or shipped with a vanilla copy.
You won't be able to (easily) repackage a legacy update file, because they're just a bunch of delta patches stacked on top of the other, and messing with that process manually is... ill-advised

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But it is technically feasible, via KindleTool.
I do have a K2i on FW 2.5.8, but I don't know if it was using the exact same config files as a DX, so that may not be of much help, unless you can get me the expected checksum, which should be jotted down somewhere in the mess that are those legacy update scripts (again, KindleTool)

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