No certainty, but the probability is high that these would be B&W slides contact printed from camera negatives...just because that would be a lot easier. Identification hinges a lot of the actual age, too. The first roll film base material may not have been stable enough to use in the high heat of the magic lantern shows, so might have been contact printed onto glass instead of more roll film. Just guessing here.
Originally Posted by Grainy
What is interesting is that the size of the piece of class is about the same as a microscope slide