I wondered if one of the sub-folders might have something at either this
location....
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\E xcel
or possibly at....
HKEY_USERS\S-1-5-21-507921405-1958367476-1801674531-1003\Software\Microsoft\Office\11.0\Excel
(possibly in the UserInfo folder, where a hex string is stored)

Simple test: first export this particular registry key to be able to restore
it in event of catastrophe, then delete the value, and see what happens! If
the data is deleted from the drop-down menu, voila; would have to check that
only the unwanted data had been affected, and then re-establish new data as
you want it. A project for a quiet aftenoon....