[Mcgyver]

i would say decades ago much like they do today, accuracy from high quality ground screws, on grinders driven by leadscrews.

the question remains, where did the first decent leadscrew come from. flats and squares can be generated with simple equipment to a very high degree of accuracy. Not sure how this is accomplished with screws, as the common machinery used to make them is of course driven by a screw. Before screw cutting lathes (ie that had a leadscrew) threads were laid out by hand believe it or not and cut with tools like a graver (possibly followed up with a chaser?)

keep in the mind that the leadscrew or feedscrew isn't used as an absolute reference that often over these distances, for example you wouldn't rely on the feed screw to hold a dimension over 20" on the lathe or mill for highly accurate work, you'd get close and then measure with a mic (who's accuracy is also based on a screw but is adjusted to a reference)

is this going anywhere close to helping? not really sure what you're after

PS, don't forget, the airplane you ride in, the nuclear reactor providing electricty and the the space shuttle are all more than 2 decades old - we've had the ability to do accurate work for a long time, heck, how long have jig borers been around? probably fair to say though technology through cnc has made accuracy a lot easier and accessible.