Just to avoid confusion, and there's still plenty of it, everywhere, the above sentence is correct -- but those marbles are not now called "Leighton." At least not by a lot of people. They're usually referred to as German hand-gathered "ground-pontil" marbles -- the marbles formerly known as Prince. Leighton. And the bases are frequently light-colored and transparent.
Those of us who have made the adjustment (and not all have) reserve the term "Leighton" for those American hand-gathered, melted-pontil marbles actually made by or closely associated with J. H. Leighton (Navarre, Barberton, etc.). Lots of them are purple.
And restrict the term "transitional" to marbles that were hand gathered but machine rounded. Still some discussion on that one, I think. But it makes sense.
Although I think a case can be made for calling the earliest hand-gathered slag-type marbles transitionals, too (like the ground-pontil Germans and the American Leightons), as transitioning away from the traditional cane-cut marbles. But then would you have to think of all hand-gathered marbles as transitionals, whether machine-rounded or not? Because they're all eventually to be replaced by machine-mades?
Ugg. Maybe another thread.
Making a long story short = every time you see the term "Leighton" or "transitional," little tiny warning flags should pop up. Find out what those terms mean to the person who's using them . . .