I just tried an experiment, which may solve this mystery.
I converted several books in Calibre into MOBI (dual-format, or Hybrid). I also used KindleUnpack to unpack the hybrid MOBI generated by Calibre, so we could compare this to the file downloaded onto the Kindle later.
I then used OSX Send-to-Kindle to upload these to Amazon.
I then downloaded them via WiFi onto my Kindle Voyage.
I then plugged my Kindle Voyage in by USB, and looked at the books - it would appear that *some* books were downloaded as AZW and *some* books were loaded as AZW3
Book 1 is an example of a book that was sent as an AZW.
Book 2 is an example of a book that was sent as an AZW3.
I've attached examples of each book at each stage:
- Original Source AZW3 Book
- Hybrid MOBI Generated by Calibre
- Split up by KindleUnpack - MOBI7 component
- Split up by KindleUnpack - Mobi8 component
- Copy of book downloaded onto Kindle voyage
It should be fairly easy to do a comparison here to prove what happened.
I am quite curious why some books got uploaded as AZW and some as AZW3 though.
And also not sure how the cover images (thumbnails) got broken in the download.
@speedlever - As in, books I upload via Sent-to-Kindle will appear under "Docs", as opposed to "Books". It's just a logical division used by Amazon - I believe it's there on any reasonably modern Kindle (PW, PW2, 7th Gen etc.). I'd like a way to get the books I upload to appear under "Books" - apparently I need to do some mucking around with some 501 CDE tag, I was hoping there was a reasonably easy way to do this from within Calibre.