This is just a possibility.........
Some years ago (when we were researching the differences between Northwood and Fenton enamelled water sets) Steve and I noticed that several of the Northwood tumblers had moulded numbers on the base 1, 2 and 5 were on ours). We discovered that these numbers (and letters were also used) were possibly to keep track of a workshop's output within the glass factory. Possibly the 6 was underlined so that it could be distinguished from the 9.
It's just a thought. I could be way off with this in your case, but I thought I would throw it into the ring as an idea.