Looking over your post again. The description Aoi Art provided was "HS Code 9706.10 (Antique Japanese sword, made more than 250 years ago)
This isn't adequate and also missing the subcategory.
It should have at least contain: "Antique Japanese Sword (Katana), 1575, Bizen Province, for collector purposes only. Not a weapon"
Correct HTS Code is: 9706.10.00.60
Documentation should include a copy of the NBTHK paper and anything else that indicates year made.
US Customs have a reason now to be overly strict on their classification, so using the right info is necessary.