Thanks, Bruce. I guess I never really paid much attention to that section since I am not real big on hand-made marbles. It's interesting to me that they were sized in 1/8" increments, except the difference between a No. 0 and No. 1, which was a difference of 1/16" and the increment between No. 8 & No. 9 & No 10, which were 3/16"each. Honestly, the sizing system described seems a bit arbitrary to me, but I suppose it helped cover the bigger sized ones.
The American machine-made system is much more standardized. As far as I know, it uses 1/16" increments throughout (No. 00 = 9/16, No. 0 = 10/16, No. 1 = 11/16, etc.), which makes it much more sensible, at least to me.