@maskman
You can find serial in Device Manager. Connect Kindle and open device manager, search for "USB controllers" group, and than find "USB mass storage" or like it. Open it and go to third tab. You can now select USB parameters and view it values. Look for first value like "USB\VID_1949..." at the end of this string you'll find Serial number.
See screenshoot, it's in Russian, but I think it will help you:
http://clip2net.com/s/O2Om