Annual Report: A corporation's annual report is usually a colored, professionally presented, and marketable document intended for an audience of shareholders. 10-K: The 10-K report is submitted to the SEC in a generic format, with no pictures or charts. The SEC has very strict guidelines on what information must be included and how it must be organized. Source: https://www.investopedia.com/ask/answers/102714/what-are…
From then on, they're 'annual report's.
I'm not familiar with US forms, but perhaps it's possible there used to be a vaguer reporting requirement called '10K', and the format had more freedom, but then from 2003 it was just a form to fill out separately to a flashy annual report to investors?