ok, I checked my stuff last night and can add Luxembourg, French Polynesia, Thailand, and Mexico to the list. Luxembourg and french Polynesia may be balls but if so, they are very small balls or big marbles, who is to say? Mexico is an easter seals release and thailand is also some type of charity but both are stamps with marbles. I know that both Mozambique and England had two series with marbles on them. there is also a FDC out there from the american glass that pictures german handmade marbles on the cachet. (go figure). As far as games played with marbles, both Macao and Sweden have stamps depicting what we call chinese checkers, and Djibouti and Gabon have stamps with mancala on them. Brazill also has a nice FDC with kids playing marbles.
Steve