Unfortunately my version of MS Office does not contain the Century School BT so I cannot see how many files are associated with it. But a lot of fonts have many files. Arial for example has quite a few, including Arial Regular (arial.ttf), Arial Narrow (arialn.ttf), Arial Bold (arialbd.ttf). Is it possible that Century School BT has a narrow (light) version and you accidentally copied that file into the Kindle font folder as Serif_Regular.ttf?