Getting back to the dial on the back of that printing frame...
...Lessee, the numbers run from one to twelve. Why those numbers? Is this device for forensic printing, so you can keep track of which copy of Exhibit A goes to which juror? I don't think so. What other photographic processes back in the late 19th century might be relevant to numbers 1 through 12?
Aha, how about... hours of the day? Way back then, a lot of contact printing was done on printing-out paper, which didn't require development, but which DID require hugely long exposures, often on the order of several hours. The way you'd determine correct exposure was to load the print frame and lay it out in the sun; then, periodically, you'd come back, open one part of the back of the print frame (this is why they were designed to be opened in sections) and take a peek at the printing-out paper to see whether you had enough exposure yet. (You couldn't do this from the front because the negative was in the way!)
But during a busy afternoon of printing in this way, think how easy it would be to lose track of how long you had exposed each print. Solution: a little ratcheting "exposure time reminder." Put out your print frame, come back an hour later, check it, and then advance the dial a notch (that's why it's handily located next to the removable panel, so you won't forget it.) Check after another hour, advance the dial again. Why do the numbers only go up to 12? Because after 12 hours, it's probably going to be night, and no further exposure is possible!
Anyway, that's my theory. I'll pass on the duck, unless it's a l'orange...
