I'm going to try various downloading options. I've got several older Kindles here. Maybe one of them is old enough that Amazon isn't updating the firmware and it can't handle AZW6. Perhaps downloading to one of those will give me an AZW3. I'll also check the Kindle for PC route, here.
As far as the unknown extension, I'm not knowledgeable enough to comment. About all I know is I put the electrons in here, and a different set of electrons come out there. If some of them want to be unknown, well, that's just part of the magic that Calibre does.
