The Antikythera Mechanism is an early analogue computer, discovered in a Roman shipwreck off the coast of Antikythera. It was ridiculously advanced for its time, being able to calculate eclipses, chart planetary positions, and convert between the solar and lunar calendars using a system of bronze gears.
Some fragmentary inscriptions (scroll down a bit to see them) were found along with the mechanism, possibly a sort of "instruction manual". They were definitely written in Greek—but what sort of Greek?
It's thought the device was made somewhere around the second century BC, so the obvious answer is Koinē. But other dialects definitely still existed at that point; I'm curious if the inscriptions show any hint at them.
(A better transcription can be found in the supplement to Freeth et al, "Decoding the ancient Greek astronomical calculator known as the Antikythera Mechanism", 2006. But that's not freely available, while Wikipedia is.)