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I didn't expect very much from it but actually it's quite accurate and good power supply after one modification and calibration. At the beginning analogue voltage meter had about 0.5-0.7V offset. That may even break things especially in low voltages (for ex. 4.0V instead of 3.3V).
Datasheet says that it have <1.0mV load regulation @ 15V output, <1.0mV line regulation @ 2A load current and ripple voltage should be under 0.3Vrms. Sense wires are also quite nice addition.
After some tests I opened it up and found a few calibration potentiometers. None of them appeared to affect volt meters offset until I found a place of potentiometer that had been replaced with ordinary 81k ohm resistor (series with meters coil in voltage mode). That was quite easy to replace with a 100k potentiometer and now it seems to be bang on!
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Considered also beefier components to zener regulator that seems to get really hot.
..but then thought that if it have worked that many years without any problems, it won't break anymore :)
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Edit: One of the readers asked about PCB pictures. Download here