Here, at 1:02, I was surprised to see Finbar Furey, when going from High B to a half-holed High C natural, rather than put down an anchor finger, actually grasp the bottom of the whistle with his lower hand. (He’s playing a Mezzo G whistle of the Bernard Overton style.)
Finbar Furey : The Lonesome Boatman (2012) Live Irish TV
BTW the original 1969 recording seems also to have been done on a Mezzo G whistle. If it’s true that Bernard and Finbar developed their Low Whistles in 1974, one wonders what whistle Finbar used in 1969.
Interestingly on the album Best of the Fureys: Spanish Cloak Finbar is using an Ab/G# whistle.