1950's. There are plenty of extant photographs of them in WW II. It
does not take a jet to make a contrail. Contrails have been around for
a long time. They have always looked and behaved the way they do now.

If anything, the contrails from the old WW II era piston engine bombers
and transports were much more sinister and deserving of being called
'chemtrails' because in those days, the aviation gasoline they burned
contained copious amounts of tetraethyl lead and the exhaust from
those big radial engines contained a lot of poisonous lead ash.