Pete,
the flat part of the top of the rear toggle link doesn't seem to have ever had any s/n. And the machining on the knobs look very "authentic" to be a late, non-original addition; weird...
http://forum.lugerforum.com/lfupload/dscn0331cd.jpg
http://forum.lugerforum.com/lfupload/dscn0327a.jpg