if they are real my bet is that the moment we started doing nuclear reactions in the lab is the moment we gained notice. On December 2, 1942, the world’s first self-sustaining, controlled nuclear chain reaction took place.
looking at that nuclear reaction as the starting point I would calculate out the speed of light and then look at all the star systems. within the range of the time light travels across the universe and the date of a downed UFO to get a location for the alien race.
My bet is that getting here is difficult and whatever crashed could of been a robotic probe deployed when the nuclear reaction was detected.
After that 1947 date I would guess that eventually something more then a probe showed up.... of course, this all is said with the idea that it is real and did happen. seeing how all of this is over 75 years ago during the cold war I can only guess if any of the crashed UFO story is real.
The nuclear explosions changed their visitation patterns. They have been with us for a lot longer than that.