Hi Ken,
Sorry, I'm not going to be able to tell you too much about your sword, but lets start with what we can see. The sword is a real Japanese sword called a wakizashi, traditionally a sword between 12 and 24 inches. For Samurai it was a companion sword carried along with a long sword (katana). Merchants and non-samurai could carry a sword of this length, but not a katana. The sword was carried edge up inserted through an obi. I'm guessing it is a Shinto period sword. That is, a sword made between 1596 and circa 1780. The condition of the polish is not good, so no details of the grain or hamon can be seen to help us evaluate it. Hawley lists only four Tadayuki working in Higo or Hizen, all between 1661 and 1845.
Perhaps one of our better sword evaluators will weigh in, and can provide a better appraisal for you.
Hope this helps, Tom