Possibly stupid suggestion; but is Horst Heussner still alive, and if so might it be worth getting in touch with him and asking if he could give you some more info on his sources? It's not completely implausible that he might be interested in talking to someone about any changes/additions he made to the edition he was working from, and he might be able to point you at the right library. Then you could talk to your Viennese acquaintance about going to have a look. (Actually, if your Viennese friend is unwilling/unable to help, I know some people in Vienna, and if you can turn up any specific leads I'd be happy to ask them if they know anyone who might have access.)

(Edited to clarify something I didn't explain properly.)