Referring to zakpijp and pypzak as being mentioned "in a few 18th and 19th century dictionaries", Boone fails to reveal his actual source, because this spelling doesn't occur at all in his note (➺ p.93, note 54); NB¹: Pyp-zak is defined as "doedelzak, cornemuʃe (f)." in Pieter Marin, »Nieuw Nederduits en Frans woordenboek« (Dordrecht, 1773; 8th ed., revised and enlarged by Ernst Zeydelaar); NB²: Note the quotation "Op de Pypzak speelen. Jouer de la cornemuse" in WNT lemma Pijpzak; The reference to Marin results in a list of 11 editions, but I was unable to detect it in the above mentioned edition of 1773 (which to my surprise doesn't occur in the list shown in WNT).