What device/component from the early 1940's can listen to only a specific narrow range of acoustic frequencies?
Sonar history lesson: -


  In 1906, American naval architect Lewis Nixon invented the first
  sonar-like listening device to detect icebergs. During World War I
  (1914-18), a need to detect submarines increased interest in sonar.
  French physicist Paul Langévin constructed the first sonar set to
  detect submarines in 1915. At first, these sonar sets could only
  "listen" to returning signals. By 1918, Britain and the United States
  had built sonar sets that could send out, as well as receive, sound
  signals. The U.S. military began using the term "sonar" during World
  War II. As with radar, new military applications for sonar are
  constantly being developed. For example, in the early 2000's, the U.S.
  Navy introduced a sonar system to help clear military mines.


Text above taken from here. Use of resonant circuits whether mechanical or aided by electrical filters was well-known throughout the 20th century.

US patent 609,154 made in 1898 describes the use of matched tuned circuits used in a radio transmitter and receiver. I think Tesla may have made an earlier patent regards the use of tuning. Tuning/resonance was therefore a fairly open-book since the start of the 20th century.

Piezo electricity: -


  The word ‘piezo’ is derived from the Greek word for pressure. The
  piezoelectric effect was discovered by Jacques and Pierre Curie in
  1880. They found that pressure applied to a quartz crystal creates an electric charge in the crystal, a phenomenon they referred to as the
  (direct) piezoelectric effect. Later they also verified that an
  electric field applied to the crystal leads to a deformation of the
  material: the inverse piezoelectric effect. In the subsequent century,
  research has been performed into the development of materials with
  improved piezoelectric properties, enabling commercial utilization of
  the piezoelectric phenomenon.


Taken from here. Making a mechanical movement resonate has been known about far longer than just the 20th century - think tuning forks and pendulums etc..