The medieval number system was invented by the Cistercian order in Europe in the 13th century. Cistercians are a Catholic religious order of monks and nuns. They used, amongst themselves, a secret number notation. No one really knows about it until about 20 years ago, a British orientalist and historian David A. King wrote a book about it. It was used by various different groups for kinds of medieval cyphers and secret number systems. It is a system that can only write every number from 0 to 9999. It can not to any other numbers. It is really easy to use for dates and page numbers of books.
If you are interested in these numbers watch this Numberophile video.
Great project, love the hand-carved keycaps. Just wrote this up for the blog, should publish in a few days. I also made this a featured project -- everyone should see it!