RCA Carbon Microphone
RCA Carbon Microphone
This double-button carbon microphone was introduced by RCA circa 1935 primarily for home recording use with their high-end radio-phonograph combinations.  

The one shown above in the bronze colored painted finish is the most often seen.  There were black and gun-metal versions as well.  Unlike many of its contemporaries, the microphone element was not suspended on springs.  Instead, RCA relied on short lengths of rubber hose for noise isolation, as can be seen below.

1937 RCA Microphone Catalog Entry
RCA Carbon Microphone-Covers Removed
Some of these microphones have a glued-in RCA/Nipper logo.