The Neumann SM2 is a stereo valve ( tube ) microphone
Neumann’s first stereo microphone. With its two independently switchable small diaphragm systems (omni, cardioid, figure-8) the SM 2 allowed for all kinds of coincidental stereo recording techniques.
If you visit the Neumann web site HERE you can download various articles about the SM2
My very good friend Dan Alexander has recorded an excellent video about the SM2 for 'You Tube' but unfortunately there is a problem with the sound, in the meantime you can see it here and if you are very quiet you will be able to hear it, Dan is trying to sort it out. I first met Dan in the 1970s when he was buying up hundreds of valve mics in Europe and taking them back to the USA.