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I know this won't help now but what I do if I'm buying books from Amazon for instance and download them, I have a SOURCE folder of all my ebooks whether they have DRM or no DRM. I then drag those into Calibre and Calibre then imports it and creates its own folder with that source file.
Sometimes I have done something similar where I've imported a book, converted it, deleted it and then dragged the converted back in so lose the source, so the above workflow for me is great because it means I always have a copy of the original file somewhere.
In terms of how you can determine which is the source, the only way I know of is by date stamp. But if you've done like I have then that may not even be the true source version of the ebook and may be a converted version.
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