Frit marbles are made yet today--by many marble companies throughout the world. The first ones were made by folks actually standing next to the rollers and throwing/tossing/spreading glass bits--or frit on the hot, out of the oven marble on the rollers. I am sure that this has been refined a bit--at least I hope that it is.
They are a very hard marble to ID unless you are like Al, with some original packaging.
The most desirable are the ones that Vitro tried--they are rare and without proper history or providence--I would never bid on one.
I like them--but they all look quite the same to me.
Marble--On!!