That looks like a General Electric NE34 neon glow lamp. Usually found with an E26 screw base that also houses an integrated resistance for use at 120 volts so it can be powered directly from standard US line voltages. Used anywhere a low power but long lived light source is desirable. Industrial indicator lights, nightlighting, that sort of thing.
In this case the bespoke mounting assembly suggests that one has been adapted for scientific or experimental uses, but for what specifically is a mystery. I can`t tell if the resistance is still present within that assembly at the bottom or not, but if it is, perhaps the three terminals allow for connection with or without it in circuit depending on application.
