The makers had sections the marbles rolled down. If the marble fell between the 1/2" track, it was a 1/2" or maybe reject, if it fell through the 5/8" track, it was a 5/8" marble even though it might have been a bit less. If it didn't, it would go to the 11/16 or 3/4" slot, whichever they had set up, and it would fall through it and be called that size. A .66 marble would have gone to the 11/16" or 3/4" track most likely. Some makers even used the ought system, like double ought (00), and so on. They weren't so precise as we are now with these calipers.