This is a 3.5mm Stereo PCB Surface Mount socket with a switched pass-through for one channel.
See the picture for the pin out (this was created by myself, you probably should verify before putting into use).
The pass through connects to A when the plug is not inserted, so for example you can put a "speaker" on the pass through pin, and the sound output to the normal pin, with nothing plugged in, the sound passes through to the speaker, but insert a 3.5mm plug and the speaker pass through pin is disconnected leaving just your headphones to get the sound.
Note that only one channel appears to have pass through on this model.
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