RCA MI-6204-C "Varacoustic" Microphone
This is one of the microphones in RCA's "Varacoustic" series, the MI-6204-C, from the late 1940's.  It was recommended for public address and stage use.

It is a ribbon type microphone with a unique adjustment mechanism to alter the directional characteristics according to the application.  The ribbon, itself, is just barely visible in the picture below between the poles of the magnet, and is mostly obscured by a piece of fabric screening.  Behind the ribbon is an acoustic baffle which RCA called the "labyrinth" which was adjustable via the slider at the rear of the microphone.