Hi,
I think I found the correct answer, it is the function
GetParentDirectoryFromMailingListURI(uri)
This is what I have to do:
selectedABURI = GetSelectedDirectory();
selectedABURI :: "moz-abmdbdirectory://abook.mab/MailList2"
parentABURI = GetParentDirectoryFromMailingListURI(selectedABURI);
parentABURI :: "moz-abmdbdirectory://abook.mab"
And this function is indeed doing a little more than just cutting of the
MailLIst2:
https://dxr.mozilla.org/comm-central/source/suite/mailnews/addrbook/abCommon.js
Thanks for pointing me in the right direction.
John