The set of 20 disks that represent each of the 20 centuries of the last two millennia are going to tax my ability to search the Wikipedia (since my history knowledge isn’t that comprehensive).
(Image: © Copyright Gregory Williams (Flickr) and licensed for reuse under Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic (CC BY-NC-ND 2.0))
Here we are in the sixth Century, so the years 500 to 599.
The Roman numerals DXXV at the left convert to 525, so, what happened in the year 525 AD?
The answer, strangely enough, is “AD” itself:
The denomination 525 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years. In this year, the monk Dionysius Exiguus proposed a calendar starting with the birth of Jesus (the AD system), so this was the first time the year was designated AD. However, the system was not used in general until the reign of Charlemagne in the 9th century.
In the triangle below, the letter “n” promises to be useful, but we don’t have enough letters from this series yet to guess at any words.