The word "transitional" has ... um ... transitioned in how it is used.
Mostly now I just see it for the Japanese marbles. Which may be from the 30's. Could be a little earlier. They haven't been pinned down as far as I know.
Mostly the word "transitional" refers to a time between purely handmade marbles and purely machine-made marbles. As our understanding of marble history has grown the word has mostly been replaced by less general terms, but it seems to have stuck with the Japanese marbles.
So, like the handgathered slags from the early American companies, these would presumably have been created by a worker dipping a punty into a glass pot and then turning the glob around to pick up that tail of glass ... and then somehow cutting it, and dropping it onto the rollers for machine rounding. But I don't know how the wrinkles got into the pontils. Maybe they did their work at a cooler temperature than American companies tended to use.
So basically you have a Japanese slag, but we call it a transitional.
P.s., the official name for American slags -- the name used back when -- was "onyx".