Tenouchi 手の内 are a Kakushibuki 隠し武器 (Hidden weapon), or a Hibuki 秘武器 (secret weapon)
This Tenouchi shaft is hexagonal in shape and is tapered from the back to the front which is pointed.
In the middle a chain is attached with a round weight.
Usually Tenouchi have a cord instead of a chain.
The chain is hand made.