I'd also suggest Japanese as a possibility. For some reason that hasn't been a popular idea. There's "made in Japan" packaging, but it has been argued that it was the box which was made in Japan, not the marbles. To me that doesn't seem consistent with the purpose of country of origin packaging laws. But I'm an amateur, so ?
The mibs below aren't an exact match for yours but maybe similar enough to give pause.
First a novelty box marked Made in Japan. The marbles here were listed as 11/16".
(extra pix, click to enlarge)
Second is a box of "Bolitas" which says "Industria Japonesa", which to me indicates Japanese manufacture, possibly intended for sale in South America. From a Marblealan auction. He said they measured about 1/2".
(sorry, no clicking, wish the pix were larger but this is what we got! :-)