"the technique having nothing to do with Helmholtz Resonator, or tricky pitching just simply stopping the end-hole with your finger or palm, thus doubling the harmonic for the overtone scale, which are controlled by overblowing."

Well, that's true for THAT family of instruments, but if you are only doing 'overtones by overblows', you ARE doing something like the 'trumpet overtone series'. The instrument I own and play does not work like this. It works by changing the volume of the space enclosed by the cupped hand over the open unblown end AND doing the 'overblow series' - in fact it plays more like a Tabor Pipe (which I have also played!) - if that's not Helmholtz Effect involved in changing the resonant cavity with the cupped hand, I'll accept correction by someone who knows more Real Science than I. :-)

And I seem to remember that the maker of this whistle was "Overton". I don't have it here but seem to remember that it is stamped "G" - basically like a Low G undrilled Overton whistle body.