This is one of the microphones in RCA’s “Varacoustic” series, the MI-6204-C, from the late 1940′s.
It is a ribbon type microphone with a unique adjustment mechanism to alter the directional characteristics according to the application. Behind the ribbon is an acoustic baffle which RCA called the “labyrinth” which was adjustable via the slider at the rear of the microphone.
You can see this model used in the movie Seabiscuit by the track announcer.
