Transitional refers to being handgathered and machine-rounded. Not a transition to another style of marble.
Handgathered American slags could be called transitional, but for some reason we tend to leave the transitional name for the Japanese ones which are basically the same style of marble as the American slags.
In my mind I estimate 1930's for the Japanese version, but that's an old estimate I adopted a long time ago. I understand there is new research that may give more precise answers. Maybe someone can link that up for you.