Dual Element Microphones
This type of microphone combines two microphone elements. Figure D1 is a diagram of the Altec-Lansing 639B Dual-Element Variable Direction Microphone. Though Mr. Steele's collection does not contain this particular model, the American DR330 and the American DR332 in the gallery below are both very similar in construction to the Altec-Lansing 639B.
The five Dual Element Microphones in Mr. Steele's collection are all of the combined Ribbon and Dynamic type, as shown below.