I don't know if Heussner is still alive. I'd have to be very careful in any wording regarding asking about changes, but it's possible.

Also I don't speak German, and I don't know how his English is. There isn't a translator listed for the Preface but that doesn't necessarily mean it wasn't translated by someone at Schott: the typesetter isn't listed either, and typesetter and editor are almost never the same person (or that is my understanding).

There is another bit inside the front cover, I missed it before, that says:

"Veröaut;ffentlichung des Hessischen Musikarchivs Marburg, herausgegeben von Horst Heussner."

Possibly I need to get a German speaker to tell me whether this means "from such-and-such library".