It does resemble a Chieftain, but it appears that the beak is too short, for either the OS (old style), or NR (new range) that came a little later. You can see a picture here on Phil’s website here, in the post dated 29/04/2007 comparing the two, along with a V3.
http://www.kerrywhistles.com/blog?dir=asc&order=created_time&p=6
There are other pictures of Chieftains on the web, just do a picture search in Google, and you will see more. If you have the whistle in hand, it might be easier to tell comparing it to others.
There were always knock-offs by other small makers, of popular whistles, so I would not be surprised if it is. The D marking on the bottom is a little different too-and oriented differently as well. And if it really is chrome plated, then think possibly Indian or Pakistani. Phil always used aluminum for his silver colored whistles.
Heh, now I’m not so sure it isn’t a Chieftain. Take a look here, and you’ll see how short the beak appears in one photo (as it does in yours), but in the other, it looks longer, when the angle of the camera isn’t so great:
http://pipersgrip.50webs.com/OS.html
The D mark is still oriented differently, but who knows-that can change over time too from the same maker.
One clue that might make it easier to decide, is to how it plays. If it’s a Chieftain, it will most likely have some notable character to the tone; it will be fairly strong, with some wind noise or chiff. I don’t think I’ve played a Chieftain or Kerry whistle that was very clear toned, lacking in character or strength of voice. So that may help you, other than just picture comparisons.
Either way, if you enjoy playing it, and you got it for a good price, then play away, and worry not about its origins, unless you want to sell it of course!