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A really nice guide to the whole subject of nihonto authentication:
Origami - Authentication Papers
When you are studying, buying, or selling genuine Japanese swords, one of the most important things to understand is papers. Like most historical artifacts, or expensive artwork, provenance and documentation have a huge effect on value. There are several organizations in Japan that exist to authenticate nihonto. The most famous and largest organization is the NBTHK based in Tokyo. However, there are others that are thought of highly. As well as origami, some experts also write sayagaki, or opinions on shirasaya.