Barbara,
may I advise that you can run a search covering all three State Archives (Hannover, Osnabrück and Wolfenbüttel) at one time by clicking the link titled
"Klicken Sie hier, wenn Sie über alle Bestände gleichzeitig suchen wollen!"
in the bottom section of the page you mentioned (http://app.staatsarchive.niedersachsen.de/findbuch/)
Wish you and all list subscribers a prosperous New Year!