the thing mentioned as a tenon is more commonly refered to as a filter adaptor ,it reduces the stem drilling for a 9mm filter to a smaller about 3.5mm hole to make the stem like a normal non filter one.filter pipes are much more common here in Europe than state side.
like this
http://www.ebay.com/itm/VAUEN-Pfeifen-Adapter-9mm-fur-filterloses-Rauchen-/151178482176?hash=item2332f09200