Seems they are as I suspected, little photovoltaic cells encased in plastic. About 0.5V full sun, 0.25V in a natural lit room. I've noticed that when you first hit the leads with the test clips, the voltage jumps up to almost 3.0V. It looks like even the high impedance of my meter is enough to drag them down to 0.5V. I have to wonder what they were using to read these in the 1970s.