scr voltage regulators

Can anyone tell me the design and the operation of the circuit attached?

I'm having a hard time understanding how this is supposed to act as a voltage regulator. It seems to me, that once the SCR is latched on (by injecting current into its gate), there is no way to ever turn it OFF (except by disconnecting the input power).

There is very little capacitance between the bridge rectifier and the SCR so it works like basic phase angle voltage control of sorts.

As tcmtech notes, the SCR sees the full-wave rectified signal from the bridge rectifier, thus it will turn off at each zero crossing of the AC waveform. It then turns on at some phase-angle as determined by the control circuitry. The 120Hz pulses from the SCR are filtered by C4 to give DC.