These kind of linear supplies used to be very common. Many companies made them. You will probably find an LM723 or equivalent on the board. The main chip could be marked with a special number. They all had a similar form factor.
Yepp
ua723 + 2N3055 + driver transistor , 2 pot's (Imax ?? + Vreg), and Vout + Sense .
Only strange thing is that i can only find two rectifier diodes , not four. Wouldn't that add to the ripple & less effeciency.
/Bingo
Two rectifiers suggest it is an old style "fullwave rectifier".
These use a centre tapped secondary on the transformer with each half winding producing the full ac voltage needed to produce the DC output you want.
On each alternate half cycle one diode of the pair conducts, so it is still full wave rectification.
These were common in tube rectifier times, as it was cheaper to wind the transformer with a centre tap than to use four rectifiers.
With solid state rectifiers it is usually cheaper to use four rectifiers with an untapped secondary winding.
In some special cases, like if a manufacturer makes, say both 12v & 24v supplies, the same transformer could be used in both cases, with a "full wave rectifier" for 12v , & a "bridge" rectifier for 24v.
The saving in inventory costs may justify the cost of the tapped secondary.