FBI agents talking on field phone, 1940s.
I find this picture hilarious. Those guys don't look enough like FBI agents. That phone is awesome. Very subtle too. And I am sure they are fighting against some true menace to American society, like moderate leftists or something.
This is also the period during which the FBI took to TV and the movies, creating propaganda to serve Hoover's purposes. Some of it is actually pretty good though, particularly The Street With No Name, a 1948 film starring Richard Widmark.