@mo79: KindleGen/KindlePreviewer doesn't put an ASIN in the headers, and properly reference the thumbnail, so, yeah, it works
. The Calibre FAQ answers the first point (how not to end up with an ASIN in Calibre generated files), while my patch tries to address the second point (which seems to be moot anyway on some devices/apps if there's a bogus ASIN in the headers).