I am looking for a specific font used in oldish technical engineering docs (ca. 70's and earlier American electronic schematics and/or block diagrams), and I tried to identify it online, but had not found the font's name nor a download (either free or otherwise). Strange enough for a tech font, it has ambiguity between "1" and "I", and between "0" and "O".
Many, if not all, diagrams I saw scanned online for Apollo Lunar Missions used this font, for example. Same with many old valvular (vacuum-tube), transistor and even some oldish IC (integrated circuits, chips) schematics.
I think (but I'm not sure) these schematics' texts were made with a letter-rule, a plastic band with letters/numbers bored in it. A pen following the bores' edges would then draw the letters on paper. To keep machining costs down, these rulers normally used the same shape for a zero and an "O"; a capital "R" was usually made with a "P" plus a little slant, same slant was used with "O" to form a "Q". Both uppercase "I" and "1" were usually a single vertical line.
All corners and ending caps were obviously round due to the pen being so.
I tried all online font-guessing sites except the serif one, never saw this font has lowercase characters defined and uppercase is sans-serif. No site came up with a satisfactory match. Some were close, but were very different for specific glyphs.
This is an example of the font — and overall diagram style — for an early 70's avionics microprocessor, probably used in the F-16:
If a free download is available, I won't mind!