I agree it's unusual. I'm not feeling Akro.
Not a swirl to me. Closer to a patch. If it were a German version, I guess it might be "striped opaque."
But I don't have a guess for maker.
Love it all. Great information. Sweet marbles. You are going to miss Standing Stone so much, aren't you. I wish I could get out there to Rolley Hole. Maybe some day. Maybe you'll be there visiting. Or retire there.
Hello. Welcome. It appears to be a machine-made marble. _Could_ be mid-1900's. Could be later. More pictures may help get an ID. The larger size narrows our choices, so there's a fair chance of getting an ID with a few more clues.