Shodai seems wano originated.
Nasujuro probably brought this with him.
His horse powers though. That might be imu given.
When was Shodai crafted again?
Iirc Sandai was made by Odens dad while Nidai was made by his ancestor?
Following the formula, Shodai was crafted by Nidai blacksmith's ancestor, so years ago..
Maybe Imu sama got the sword during Ryuma's era
Among Imu and that bald bum getting the sword, I will bet on Imu
Probably whole Ryuma made Government scared and never invaded Wano was hyperbolic agenda?
There were peace treaty between Ryuma and Imu, and Ryuma had to give Shodai as part of the treaty.