I visited a place called ESL Surplus in Mansfield today. They had bought out a big portion of a place in Dayton called Medelson's, and these devices were in that purchase. I remember getting some of these years before at Mendelson's, so it was kind of neat to find more. These are some sort of unmarked light sensor. Unlike a CDS cell which requires external power (they're just a resistor,) these are actually a little photovoltaic cell inside of the plastic bubble. In bright light, they output about 0.5V. I have no idea who made them or when they were made, save they remind me of devices from the 1970s. ESL had a bunch of other things from various manufacturer's inventory lines, including old Radio Shack parts. It's not that far so I'll probably be making more trips there in the future.