The voltage regulator uses two saturable inductors to control the output voltage.
This type of circuit works on the principle of phase control and is similar to the operation of a triac.
This operating principle is not very good because it generates a lot of harmonics.
There is also a filter (probably H3) to reduce them.
Before looking in detail at the drawings, I immediately thought in Russian technology .... seeing the indications in Cyrillic, my first impression was confirmed.
Is it usable? If it works, why not? It should be checked if the output voltage is not too distorted and the filter H3 is still working.
Be aware !!!! if you check the output voltage, DO NOT CONNECT THE OSCILLOSCOPE DIRECTLY TO THE OUTPUT because this regulator is not isolated.
It is necessary to use a small isolated transformer 220V / 12V for example and to connect the oscilloscope on the low voltage winding of the isolation transformer.