Murray Leinster's "The Machine That Saved The World" (novelette, free): Human descendants from future want to help us!
Year 2180 wants to discuss (via a temporal broadcast) a doom about to visit 1972: a widespread & currently harmless germ is about to mutate into something fatal. They have a plan to contain the disaster, but need two way communications to thrash out details. Hence they send out design of transmitter Now should build to talk to Future.
But one man is smelling something fishy. With the help of electronic geniuses, he will transform the received designs into functionally equivalent but visually unrecognizable alternates. And convince the Future to build this first.
Results turn out to be very interesting.
Fact sheet.First published: Amazing Stories, December 1957.
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Related: Stories of Murray Leinster.