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.