@vql , I would put it under "other" category.
Note that if you were to report as " non-employee " or business income, there would be Self-Employment tax ( Equivalent to FICA but at 15.3% ).
However, I do recognize that this could be interpreted as "wages" because the term remuneration is used in the treaty. That would mean putting it as W-2 income ---- wages while in a foreign country is mostly treated as self-employment ( because of the need to contribute SECA -- Self-Employment Tax ).
Given the above I would suggest to use the contract / award letter from the Japanese University whom employed you. Then determine whether this was an award / stipend or wages. Also I don't know whether your US University employer continued to pay your wages in the USA and the Japanese portion was for living & incidental expenses.
