It seems that the way they sized marbles, #000 would have to fit through a 1/2" hole, so would not be any larger than that, while #00 would be any that would not go through a 1/2" hole but would fit through a 9/16" hole. I thought that was why someone decided peewees had to be 1/2" or less, because it was the smallest sizing hole vintage manufacturers used for toy marbles. There may have been smaller sizes for special commercial applications, but I don't think I have ever seen reference to a #0000 toy marble.