Almost looks like two different slags (not swirls). How do the first two pics fit with the last three? Those make it look like a hand-gathered slag, and I can see one of the first two picks fitting with the last three but not both.
FWIW, lots of translucent marbles will "light up" when backlit - it's one of the cool properties of glass. But an opalescent glow is something else and the use of opalescent glass is not restricted to one company.
The opalescent "fire" (glow) may be there but I am not really seeing it in these pics. Do you see "fish eyes" (a little clear spot/window at opposite poles) on any of these?
You may want to check out the nice post Art made about Flinties not too long ago . . .
You might be right, they sure did make similar ones, but I think the "shadow effect" at the one seam is way more common on Vitros than MKs, which is why I went that direction.
I do not have any of these but they are very interesting. I agree that the colors look identical to the "PA Dutch" but all of those I have seen are much larger, like the one you show. Have you seen "PA Dutch" marbles in this size that are the quality of the large one you're showing? And is the difference in the bodies of the small ones and the large one as apparent in hand as it seems in your photo?
It's one of my favorites from Sistersville, made during the time Christensen was operating in Cambridge - seems like Lawrence Alley was determined to give them a run for their money.
No kidding, Chuck - I completely agree. You scored a bunch a nice ones but this one is off the hook! I've never seen that sort of mottling in green before.