this circuit is posted as crystal controlled inverter and generate pure sine wave at 50Hz.
The Inverter circuit “Crystal Controlled Inverter” is build around popular flip flop IC MM 5369 (IC1) followed by few other active and passive components. IC1 (MM 5369) contains 16 cascade flip flop, which divide the input frequency 3.2786 MHZ by factor of 216. Thus we get an output frequency i.e. 3,276,800 Hz/216=50 Hz.
The circuit with the obsolete MM5369 IC has a squarewave output. Its voltage is not regulated since it is too high with a light load, too low with a heavy load and drops as the battery voltage runs down.
Many modern products will not run from a squarewave since they rely on a sinewave.
Nobody makes one like this because it wastes a lot of battery power making heat. You can make a simple 50Hz or 60Hz sinewave and feed it to an audio power amplifier that drives a voltage stepup transformer.
The proper efficient way to do it is make a complicated Pulse-Width-Modulated circuit.