A standard I have to refer to often is ISO 128 / AS 1100.101 (1992). Technical Drawing - General Principles. Which has a lot to say on the issue of Letters and

A standard I have to refer to often is ISO 128 / AS 1100.101 (1992). Technical Drawing - General Principles. Which has a lot to say on the issue of Letters and Numerals in part 4.1, I've added this as it's mildly similar, since the readability of technical drawings is also critical, it was widely read in a technical industry and that it was last revised in the same year as the OP document. It should be remembered that this standard was last revised in 1992; As a consequence, computer generated text or "Machine-made characters" don't really get a look in, although degradation through reproduction (for micro-film) is noted as an area of concern. It favours sans-serif fonts that are "uniform and capable of being produced at reasonable speed by hand" and that "Upper-Case letters should be used. Lower-case letters shall be used for conventional signs and symbols normally requiring such characters, e.g. mm, kg, kPa." The recommended stencil characters are outlined as Gothic and ISO 3098/1 Type B (aka "Font 27"). The latter being preferred by every engineering firm and operator I have come across in their published standards.