While the keycaps on the IBM PC had their letters and symbols applied through dye sublimation... and they were in Helvetica, and some ealier makers of keyboards also used actual type (or maybe Letraset) to make the masters for their keycaps, by and large the letters, digits, and symbols on the keys of computer keyboards looked like they were done on a Keuffel and Esser LEROY brand lettering kit.
Recently, I've encountered the fact that a similar style of lettering was etched upon metal by machines from a company named Gorton, and for whatever reason (like Leroy still being a well-guarded trademark?) some typefaces of this style are named after that.
Eventually, though, I've found Routed Gothic to be the best free drafting-like font for my computer. The Windows version of FontForge didn't let me reconstitute Open Gorton, but that's another story.
In any event, researching this stuff led me to take a really close look at the lettering on the keycaps of the original Selectric typewriter.
The capital letter G has a vertical line on the right going all the way down to the baseline. I suppose one could get one through flexible use of a WRICO lettering guide, but I couldn't find any lettering template that offered this kind of G, and yet the letters on the Selectric typewriter's keys did have that drafted look.
Finally, though, I did find one possibility. In relatively recent K&E catalogs, an alternate template with lettering in a style patterned after the typeface News Gothic (but not as an outline to be filled in, so the ends would be squared instead of rounded) had that style of G. However, that lettering seemed to be just a tad too condensed for the Selectric.
(EDIT: there's a major problem with this theory; the News Gothic templates from Leroy apparently didn't come out until the 1970s, long after the 1961 introduction of the Selectric - and this form of the G is even found on the keys of IBM Electromatic electric typewriters! So I need to keep looking...)
And, at that point, I also noticed that the digits in Extended Gothic looked like the digits on the Selectric, which also didn't resemble those produced by other lettering guides or templates... but they were a tad too wide.
But K&E made a fancy scriber for the LEROY system that let you change the height of letters... and so that's what IBM could have used to customize the lettering on the keys of the Selectric (and on some of their keyboard diagrams as well).
This is still tentative; I haven't found every lettering guide out there, and for all I know, IBM could even have made one of its own.
