Eighty years ago, these young men were mastering the Morse code, thanks to a battery-powered code practice oscillator shown in the July 1941 issue of Popular Mechanics.  The set used a single 1Q5GT tube and was powered by two inexpensive batteries.  A 1.5 volt dry cell lit up the filaments, and a 45 volt battery provided the B+ voltage.

The magazine noted that the country had an increased need for trained radio men for the national defense program. While a buzzer and dry cell would do the trick for code practice, the smooth high-pitched tone of a vacuum tube oscillator was much preferable.

While headphones could be used, an inexpensive permanent magnet speaker is shown, which would provide ample volume for group code practice.