As far as I can tell, these are game marbles. They have little if any value to most collectors. The left one does look like it might be MK but it's hard to see any detail on the right one so . . .?
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.