An STC moving coil mic developed circa 1935, some years after the STC 4017A microphone.
The internal acoustical arrangements were less elaborate in the 4021 than in the 4017 but the response at the upper part of the range was less dependent on the angle of incidence of the sound, which was an advantage as both mics were intended to be omni-directional and the 4021 was indeed more omni-directional at HF than the 4017.
Because of its shape it was known as the 'apple and biscuit' mic. Some non-BBC writers have claimed it was called the 'ball and biscuit' mic within the BBC - this is ill-informed nonsense!
The mic was designed to be used with 'biscuit' horizontal, but there were few studio applications for an omni-directional mic. (Though Gerald
Jackson reports that local radio used the 4021 in the middle of the studio table for 'round table' discussions - "we had nothing else!", he says.) Useful outdoors as an effects/ambience mic, indoors it was mainly used in echo rooms and as a talkback mic. As a talkback mic, it was never used as the manufacturers intended; it was either fitted flush to a panel on the mixer or used with the 'biscuit' vertical on a swan neck that could barely support its weight.