What's the function of "reactive power diode" n3 and n4?
The Center tapped Transformer M1 does have leakage Inductance   and Magnetising Inductance  .Current in the inductance means stored energy .A sharp fall in current like a fast turnoff  will make a voltage spike .Diodes n3 and n4 recover most of this inductive energy and clamp the pushpull center tapped primary winding EMF     to the B+ rail which has a low impedance at the switching frequency and its harmonics . The low impedance is usually realised with a large Bypass cap that has low reactance at the switching frequency,                                 Voltage spikes are a real problem on converters like this and the diodes are a cheap and simple way of reducing spikes.