Lieutenant Colonel's counter shoulder straps

[Akira Komiya]

So the boards were originally intended to be sewn on the uniform but were later converted to detachable style as I conjectured.
Early boards came with various different attachment mechanisms although they became fairly uniform by the time the 1930 uniforms were introduced.

[Akira Komiya]

Here is a similar pair of shoulder boards which are currently on sale at an auction in Japan.
This pair has tie strings apparently intended to go through holes opened on the tunics' shoulders and tied inside to attach the boards in a manner similar to the EM shoulder boards in the previous Type 19 uniforms.

[BTR]

Thanks for the help.

[Akira Komiya]

You are most welcome.