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KFX books downloaded to an e-ink Kindle directly from Amazon contain multiple files per book. The main KFX file has most of the book content and is encrypted. The associated SDR directory has an unencrypted metadata.kfx file that contains the book's metadata (surprise!), including the cover. There can be additional KFX files containing book resources (images, fonts, etc.).
The KFX Output plugin for calibre places everything in the single main file unencrypted. The Kindle firmware is able to make use of this alternate arrangement.
I suggest that the SS hack should obtain the cover from the book's main KFX file if the associated metadata.kfx file does not exist. The current KFX cover extraction code should work fine if it is given the correct file to operate on.
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