Makes perfect sense to me... I've got a shot I just printed, one of two taken, and I could see under the magnifying glass which was the right one... but life-size I had a hard time telling them apart. I guessed correctly anyway when I put it in the enlarger, but a notcher might have made the work less a game of chance.
John Shiu,
Glad you found the listing with photos of the opal glass that's missing ... I was wondering what the slots were for and was imagining a larger table.