This is an angle ruler for planes and chisels that have become familiar with the "Sharpening Yoku" etc.
This ruler is useful for measuring the angle of the blade edge.
This is a rare item that is not sold hard to find.

Plane (hardwood): Blade angle (29 - 30 degrees), Injected slope (8.5 - steep slope), Suitable materials (barbecue, cherry blossom)
Plane (general material): Blade angle (27 - 29 degrees), Inset slope (8 - 8.5), Suitable materials (Cedar, Cypress, Pine)
Plane (soft wood): Blade angle (25 - 27 degrees), Inset slope (6.5 - 7.5), Suitable materials (paulownia, broom tree, sawara)
Chisel angle: Blade angle (27 - 30 degrees)

The rear blade of the planes is 18-20 degrees.
When measuring the angle of the plane, it is best to measure the area where the back (flat) is exposed.