That was for the limitations of incandescent lights, plastics, and mold manufacturing limitations of the era. Printing would be a question of the optical properties of the resins. It is common to find simple light pipes with 90° angles in products now. Worst case scenario, I would make it a two piece design where the lower body accepts small fishing line. Typical colorless fishing line is basically a fiber optic wire without an optical design specification. So just insert that to route the digits for each key. It would look like an old school HP alphanumeric dot display. Glue the line on the back side of each key with epoxy, then use a carefully designed raser jig to cut/terminate each line. Finally, add a printed lens diffuser. I haven’t messed with resin and it probably would not work, but if a jig to remount and align keys to the resin printer plate were possible, I could see skipping the second lens diffuser, scuffing the surface of the top of each key before attaching each strand of fishing line, and then printing the rest of the key over top of the strands so that the light is only passing through the semi opaque plastic with no indication of character without the backlight. It would be interesting to mix my experience as a pro automotive painter & graphics/airbrush artist to mess around with aesthetics of a resin applied in stages and intermediate surface finishes. A UV cured resin cannot be all that different than the stuff I am more familiar with.