The sample above was broken - a much higher voltage than expected and it was quite sensitive to knocking at the box, and a pressure on the box created weird jumps in the voltage.
The other my three samples are better - a quick measurement shows the difference against the sticker voltage from 60mV to 10mV (after 4 hours of operating under the nominal current).
Therefore I decided to open the mystery box and look into the broken reference.
Warning: Do not do it at home!!
Thanks a friendly ham we did it in a controlled environment, wearing respirators (really)..
The red hermetic box is made of a thick aluminum, coated with a black plastic isolation from inside.
The components are potted into a shiny white extremely (!!) fine powder, almost like a smoke. I knew that already, therefore we were pretty careful with handling it from very beginning.
The white powder was pressed inside such it looked as a ceramic, when we started to dig into it started to disintegrate. Finally we washed the components in water.
There are 2 bulky diodes and the zener, plus the wire wound resistor mounted on a 3mm thick plate (aluminum), no markings on the components. There is the box serial number written on the plate as well.
See below the pictures. A real archeology, indeed