It would be a really weird Master IMO. Although, I agree it's a hard one to decipher. After much consideration and consternation I am going with an old Vienna Vitro. I think the colors and the glass fit. It's a nice marble, regardless, and that is my best guess.
More good stuff . . . you seem to be on a bit of a run lately. But the coffee klatsch with your Dad's old friends seems like it might be the best part of it.
I think I see a yellow, brown, green, flip-flop, with the yellow on one side pushed up against the green on the other. Jumpin' Jiminy man, that's a nice marble! Too nice for a Kokomo, IMO. Is that an elusive Green Sunset, maybe burnt a bit?
. . . a fellow Hoosier found another. I think Kokomo marbles are interesting and great fun to collect. No doubt, they are much less common than Pelts, but I doubt I have seen half a dozen I would call pretty. The vast majority wouldn't even come close. But that is just fine with me because I don't really care for marbles that are too flashy - unless they are WV swirls, of course.
BTW, I think the middle one in the group of three could be a Kokomo too.