I'm recording in M/S-stereo for radio documentaries and would like to monitor only the M-signal (where the talking usually takes place) in my headphones. Therefore I wonder if I can use a plug like this one? Or what would happen?
That adapter has a mono TS plug. This kind of plug will short the ring contact to the ground and only leave the left as signal. But for hearing the M signal, you need L+R, namely an adapter shorting the ring contact (right signal) to the tip contact (left signal).