I don't know when or whether Bibus was named court purveyor, either. The first forum topic you list had a description by "MLF" which included the Doppeladler mark as well. The images unfortunately are gone, but the mark will have had the same meaning. Since the mark appears on other pieces than your spoon, you probably do not have to worry that it is a fake. It would be interesting to know the dates, though--obviously before 1918!
There was a post not long ago by one of our contributors here, dolpheus, who mentioned that the A2 mark was used for Moravská Třebová, and not only for St. Pölten. Please have a look here:
http://www.925-1000.com/forum/viewtopic ... %A6#p64453
I had always been a bit puzzled as to why an item made in Bohemia would be assayed in St. Pölten, and dolpheus solved the mystery for us.