These slides have a long history of being reorganized and rearchived. There is lots of physical evidence of this on the slides themselves.
Over the years, there have been four separate systems for organizing the slides. To start, some of the slides have information about what they are printed directly onto the glass of the slide itself. (TODO example).
Next, most of the slides have aged, red-and-cream-colored paper labels stuck on them. These labels consist of one capital letter and multiple numbers. Many of these labels have since been covered up by a newer attempt at re-labelling the slides. Also likely included in this system are diamonds, in the same style of aged, bordered- red-and-cream-colored paper. We unfortunately don't have any record of who applied these labels and markers, or what any of them mean.
The system we've used to identify the slides in this digital archive, is the newer, "white paper" system. This system uniquely identifies (nearly!!) all of the slides in the collection, using the same format as the red labels of a single capital letter and then one or two digits. These labels are sometimes glued over the red labels, and sometimes taped over them. The meaning and origin of these labels is also lost to time.
(TODO what we've done to reconstruct the white labelling system, after this archival work)
We have sought to reuse the existing label system as much as possible, with one small modification: any unlabelled slide, we have arbitrarily assigned a new label starting with U to. These numbers simply start at one, and count up. Because there's no natural way to order the unlabelled slides, we enumerated them in the order that they were first digitized.