I see now what you mean
I had never paid much attention to pipe chanters before
I just remembered this from Chiff and fipple a while back (I don't think anyone will mind me reproducing it here)

SIX-HOLE PURISTS SHOULD NOT READ THIS REVIEW: THE OVERTON MODAL WHISTLE.

Colin Goldie calls me up from Germany and tells me that he is sending me a whistle. This is always good news. He told me some of the details on the phone. I was interesting, but maintained the rigorous skepticism that you all have come to expect from The Undisputed King of Internet Whistle Journalism and The Hardest Working Man in Whistle Business. And then the darn thing came. I can't believe it. This is something really new. All of you WhOA (Whistle Obsessive Acquisition disorder) sufferers are doomed, as I am. I'm so, so very sorry.

I wrote Colin an asked him to give us a little information, and I'll let you read some of that now:

*****I had a request from Brian Finnegan that he wanted a high D with an additional bottom C note, so I first thought about that. Then while talking to Steafan Hannigan he had made a wooden D/C as an experiment whilst at college studying how to make musical instruments. We discussed it and when he came to Germany last month to play at Rudolstadt Festival we met up. He brought his wooden one along and I had a look at it. On returning home I made one and it worked well. ******

Now here's me, Dale, writing again: So, let me go over this with you. This whistle has eight (8) finger holes. Seven on the usual dorsal surface and one on the ventral. This is, first and foremost, a D whistle. But, it's the length of a C whistle. The top six holes are just your basic, regular pennywhistle holes. You play them just like any other whistle. The seventh hole, somewhat offset to the right, can be covered with your ring finger to produce the C below the D note. This is cool, in and of itself, because there are a number of tunes on which it would be helpful to dip down to that C note. But, there's more: Recall that the D-major scale is D-E-F#-G-A-B-C#. Since the whistle provides the bottom C-natural note, wouldn't it be cool if...ohmygosh...you could easily play the C-major scale? We already have high C-natural which, on this whistle, can be half-holed, or played like this-- O X X O O O, OR now with THIS whistle, played like this X X X X X X x. Wow. So. All we need now is a F-natural. So, there is a hole on the bottom surface, to be covered with your thumb. (The position is a little further down the shaft than where I normally put my thumb, but I adapted readily). To finger the F-natural, the 7th and 6th holes are open, 1-5 are closed and one takes one's thumb off the little special F-hole. Colin picks up our story:

******The thing with Irish music is that there are tunes which are normally very hard to play on whistle or even not possible because you do need the bottom C. You can get away with half holing the F# note to give you the F but having a hole for it makes it easier. The hole for this is on the underside of the whistle between the 5 and 6 hole. This may sound awkward as the thumb is lower down than it would normally be but with a short practice, it is easy to get used to. I have made a few of these now and also an Alto G/F model. This is much more of a stretch for the bottom hole. This will be a new opportunity for players to get more out of the whistle. I can also make the D whistle without the F hole although if the player does not need to use it, it can be taped up until required. The whistle in this case plays exactly the same as a normal whistle but having the opportunity for the C if needed. As Steafan had the initial idea he wanted it to be called the Modal Whistle which it now is.*****