Hi dandow and all,
From October 1st, 1907, the first weight-unit mentioned by Parcelpostguy was 20g and the additional unit was (also) 20g in Japan. The change of the additional unit fee from 10 Sen to 6 Sen was the main reason for the issuance of 6 Sen Kiku (chrysanthemum) stamp on Aug.20, 1907.
Below is an example of similar franking for double uprate first class registered foreign mail, using 6 Sen Kiku stamps (e.g. 10Sen + 2 x 6 Sen + 10Sen registered fee).

Hope this helps !
- Hironobu (from Osaka, Japan)