As a former Nook user, I have many books with both an AZW3 (or MOBI) format that I bought from Amazon, as well as the EPUB I converted to in order to put the book on the Nook.

Now, with a Kindle, I have EPUBs that I bought from B&N which I'm converting to AZW3, so as with the Nook books, I have both AZW3 and EPUB formats.

If I'm doing a reconversion (to fix paragraph spacing or something), I always want to use the original format. However, with the scenario outlined above, the only way I can know the original source is if I remember which store I bought a book from (or look it up in my purchase history).

Is there an easy way to determine which format is the “original”, by which I mean the first imported into Calibre? Basically, given a book with an AZW3 and EPUB, I'd like to know which came first.

If there's no easy way in the UI, is it a safe assumption that, in the “data” table in metadata.db, the entry with the lowest primary key value is the original?