The 4033A was produced as a direct result of the needs of the 'talkies', which required a microphone that could eliminate unwanted sound from the camera and other noises off. It contains two elements, a ribbon and a moving coil (similar to a 4021 minus the biscuit). The microphone incorporated a screwdriver-operated switch that allowed either or both elements to be routed to the output. The switch was marked P. R. C.
P - (Pressure) - omni directional - moving coil only.
R - (Ribbon) - bi-directional (figure of 8) - ribbon only.
C - (Cardioid) - uni-directional - combining both elements.
The 4033A was somewhat of a compromise as it tended to be "toppy" in all three positions. Nevertheless, it was the standard TV Boom microphone for many years and was also used on Radio OBs where its cardioid response was useful for rejecting colouration from PA.