Before there were Nixie tubes, there were edge-lit displays: "Each digit panel has a tiny incandescent lamp associated with it that lights when that the numeral on the panel is to be displayed. When the tiny lamp corresponding to a given digit panel lights, the light is injected into the edge of the plastic panel. The engraved area in the plastic causes interference with the light as it travels through the plastic, and some light is refracted out through the plane of the panel, causing the engraved dots making up the digit to light up with a white glow. The resulting digits look much like the fully formed numerals in a Nixie tube, except rather than an orange glow, the Canon display digits give off a cool white glow like that of an incandescent light bulb."
As Chris notes, "Nixie tubes are cutting-edge compared to this."
Non-Linear Systems Model 481
Edge Lit Display Teardown
[Old Calculator Museum]
(Thanks, Chris Hoppe!)