Hello I noticed the following ebay listing:
http://www.ebay.de/itm/263129114566The device is fascinating in several ways. It is so old it does not even have nixie tubes but uses twelve bulbs per digit instead. Of which there are only four, btw which fits to accuracy spec of 0,1%.
The voltage standard is a (unsatured) weston cell, which does seem to work still despite its age.
The "check cell voltage" does work. On my request, the seller measured an AA cell with plausible 1,589 Volt.
The seller will not ship the device, as the weight is 22Kg, so this will be of interest only to the local people.