Clarke whistles have two main types:
The “Original” and the Meg/Sweetone/Celtic.
The “Original” variety is a rolled tinplate cone, the seam soldered at the back, with the blunt-end flattened on sides and bottom around a cedar block - the heart and soul of simplicity. The windway is formed by the gap between the block and the tinplate, the labium/sound-blade is formed by the lower part of the sound window pressed downward into the airstream.
In its off-the-shelf form, the sound of the original is usually way too chiffy and requires a lot of air to sustain a note, with a bit of experimentation with the height of the windway and level of the sound-blade, they can be quite nice. A lot of players appreciate this little cheapie because it makes you work hard to get a result - kinda like doing weights.
The Meg/Sweetone/Celtic variety are almost the same whistle - it has the same rolled tin-plate cone as the original, but has a plastic head instead of the cedar-block arrangement. Sweetones and the “Celtic” are manufactured in a British factory while Megs are manufactured to specification off-shore. The British-made ones seem a little more stable. Megs/Sweetones are both fairly quiet - good for learners, small sessions or if you wish to play-along without being prominent.
Legend has it that the Meg/Sweetone plastic head was designed by Micheal Copeland. It is much more consistent than the Original, but not a lot of opportunity for improvement - what you get is what you get, Jerry Freeman does a reasonable set fo tweaks and that’s about it.
The Original seems to have more note-stability, depending how you tweak it, while the Meg/Sweetone allows for a high degree of pitch-change by using breath-pressure - this can be an advantage, but can be hard for a learner to keep on-pitch.
With these whistles - if you know what you are dealing with can be a perfect instrument for learning or performing.
In all these, the D is usually in good tune, while the C can be a bit of a trial to keep in pitch.
Regarding the “Celtic” - it is a sweetone with a heavy enamel finish. In my experience the Celtic plays better than the standard Sweetone - more stable over the octaves and a slightly broader tone. This could be just subjective - but that bit of paint does seem to make a difference.
Hope this helps!