Apparently your device has a new USB identifier. The previously reported dell devices have had a rev_ of 0100 and 0224, while yours is 0226.
Vendors change these without notice or reason, which is incredibly frustrating for people trying to support them.
Request support for your (new) device at https://bugs.launchpad.net/calibre
, providing the information the FAQ entry http://calibre-ebook.com/user_manual/faq.html#id20