For Information:
In the meantime I bought exactly that car with the original documents from 1959.
Dezember 2012 I started a public question for more information in the "Oldtimer Markt" magazine in Germany and talked to a few surviving veterans of the "Waggonbau Bautzen".
They told me, that the directors / technical directors ... of that plant planed to build 7 cars, but had to stop that projekt because of political problems. The exactly number of cars built is not known, but I found no further car.
The history page og the "Waggonbau Bautzen" got some pictures and information from me for their homepage.
Kaimann