I'm not 100% sure about whether my information is the thing you're looking
for, but I think it should be of help.  The history of the GDR page
(www.ddr-im-www.de) has the following:

   "[... der] Weber-Nachlass wird vom Urenkel des Komponisten, Hans-Jurgen
   Carl Maria Freiherr von Weber, an die Deutsche Staatsbibliothek in
   Berlin (Ost) ubergeben."

Roughly translated: "The Weber legacy is given to the German Statelibrary
in Berlin (East) by the composer's great-grandson Hans-Jurgen C.  M.
Freiherr von Weber." This happened in 1986.

If I'm not very mistaken the possions of that library went into the Berlin
Statelibrary after the reunification.  That library can be found at:

   http://www.sbb.spk-berlin.de/

I'm afraid it's German-only, but I'll gladly help you contacting them if
you should need so.

You might also find of interest - if you don't know it yet - this article:
http://www.idrs.org/Publications/Journal/JNL14/JNL14.Waterhouse.html, which
is titled "Weber's Bassoon Concerto Op. 75: The Manuscript and Printed
Sources Compared"

P.S. Is it common on this list that new members introduce themselves? If so,
I'll do that immediately.

Warm regards

    Jan