Ian, I really thought you had the answer there for a moment and perhaps you still have if there was a variation in the standard for lantern slides. I have tried to fit a few in and they are too large by about 2 mil. on the shorter dimension. It does seem like the most likely answer but I'm really hazy regarding the process whereby lantern plates were created. If they used their own plate holders were they used in some sort of reducing back on a standard quarter plate or half plate camera do you think?