The original AZW3 would be the one Amazon provided rather than a converted copy of the original.
As for the AZW vs. AZW3, Kindle for PC doesn't have very friendly filenames
but when importing into calibre the filename will be recreated from the extracted title metadata, and AZW's will receive their true and proper file extension.
There is no such thing as AZW. It is just a renamed MOBI (mobi7) or AZW3 (mobi8).
If the original, from-Kindle-for-PC file has calibre* classes, then that particular book was
created by the author/publisher using calibre.
Most likely not by a Big Publishing House.
