I cannot tell how old my Unigor is, however I do have an old catalog of a long gone German eletronics supplier printed in
1976 and the meter is in there. If I remember correctly price was about 1100,- DM, or about 550,- EUR in todays money.
The 6e is a proper electronics meter, I still use it when I need to measure extremely low voltages/currents as it's smallest range
is 1mV/1µA full scale.
The Unimer 31 looks very much like the later analog Metrawatt meters
I disagree that BBC/Goerz/Metrawatt meters were of bad quality. You could (probably still can) find them in
most school physics labs in Germany and they were not treated very well there believe me.
I cannot comment on clones like the Iskras however.