Not sure about a slag, it might be a transparent swirl of some sort. Your first pic has the best focus and comparing it to the others, it seems you may want to get just a little farther away from the marble to get a good pic.
I didn't think marbles dated - they're inanimate objects. 😁
But a good place to start looking for manufacturing dates is in the "Timeline" threads in Steph's Study Hall.
🙂
Just a tip - to get the most accurate measurement you need to move the marble away from the ruler part of the caliper. This reading is probably about 2 mm shy of reality.
The typical old ones have a patch and two equatorial ribbons so the poles are different colors. The new ones just have two equatorial ribbons so the poles are the same color.
What happened to my Cat is right Art, that marbles's a mess! And glued back together? Someone probably thought it was a shrunken core Onion Skin or some nonsense -looks Asian to me but I don't think I've seen another that large so I'd be thinking Vacor too.