i used to hang around with a whistle maker, who often said that there is no such thing as a tuneable whistle, just a whistle in tune.
he claimed you could get all the notes in tune by a combination of moving the fipple--or on a clark you move the plug i think--and then carving out the holes further up to get what you want. this takes a bit of trial and error or a manual if there is one.
he often experiemented with various carburators in the barrel but came to the final conclusion that just getting the right size holes and setting the bottom at the right palce would get the best result. his tuned whistles were in local demand but his health left him and i don't know if he has made one for years.
he made one out of a sky pole--an a--that sounded great. he often looked for scrap tubing to fiddle with.
he was unconvinced by the idea that many have of hanging wires from the holes into the whistle--with a little solder, or with the temperature of the whistle being much of an issue--which was often locally discusses.
many year ago i met paddy maloney after a chieftans performance i went to with my daughter and he did exactly the same thing. set the low note then altered the highr ones by carving the holes bugger or smaller--well smaller is a trick but if you start with a blank barrel--you get my meaning.
he often said that just ptting a generation fipple on an oak body made a better whistle than either alone--of course tuning the bottom note. and he loved the old hohner barrels--he'd knock off the fipplke and make his own. thay had good tone although few found out as they had a very bad fipple. the old hohners also have a low g which is handy sometimes.
among the cadalac makers he liked o'riardan's, but i think he's too old to make them now, but he wasn't offended by copelands, just thought they needede a little tweaking.
there was a whole local cult around these whistle alterations which i got a great kick out of listening to, but i play harp and mandolin family instruments so i was purely a voyeur.