I found this delightful and informative marbles article from a magazine called Outing, published in 1901. It was written by James L. Steele and goes on for several pages talking about marble lingo and lore. It begins like this:
Last December there appeared in the New York Sun an innocent- looking letter from me on the subject of marbles in which I referred incidentally to marbles as "mibs," and in a few days my breath was nearly taken away by the vigorous manner in which other correspondents fell to arguing the question whether I should have said "mibs," or "migs."
For what it's worth, I'd say "mibs" has won out as of 2009. Here is the direct link to the article in Google Book Search.