Some hypothesize the time correlates pretty closely with the detonation of the 1st atomic bomb. Understanding, and applying, atomic fission would be a notable step in a species scientific advancement. This 'tripwire' event was monitored by aliens, as the theory goes, who have since dropped by to observe.
There were prop-driven planes which could hit 400 or a little more than 400 at the end of WW-II but none of them cruised much more than around 250. First jets could cruise at 500 or so and around 47 - 50 you’d have been seeing F80s, captured 262s, and skunk-works versions of whatever Germans had in blueprints as of early 45 which included swept-wing planes which could likely hit mach, at least in a shallow dive. People flying around in dc3s would have been seeing planes moving at three or four times their own speed.