Japan was allied with the Entente during WWI, but its Navy mainly kept the German navy out of the Pacific. There was little land fighting and the supply of Type 38 rifles was more than adequate. But Japan had thousands of the obsolete Type 30 rifles ("the hook safety Jap") in storage and sold almost all of them to England who later passed them on to Russia.
AFAIK, the rifles sold to Britain were standard rifles taken from storage and had no special markings. Whether the "mum" was marked out, I do not know, but I would think it likely. (The aforementioned "bullseye" was put on at the factory, in lieu of the "mum".)
The one I have, Tokyo Arsenal #204, has the "mum" overstruck with the Tokyo Arsenal symbol, not ground, so it was probably not one of the rifles surrendered after WWII. Whether it was one of those sold to Britain, I have no idea, but it has no English marks.
Jim