Let me explain the results of my tests on the two sample heads sent by Tony Dixon to compare vs. the two Trad D whistles I own; first a summary then details.
Summary: The early Dixon Trad D had a head different from the later production models as Tony has stated in the email quoted earlier. Tony sent me two old style heads to test. I fit one to the later of my 2 Trad Ds, which then sounded much like the older of my Trad Ds. The old style head appears to have been cut from a polymer non-tunable D, DX001. In the middle of this email exchange I had actually bought my own DX001. This is the one I now recommend to beginner whistlers as best "bang for the buck," if tuning is not an issue.
The Gory Details: I received two D whistle heads from Tony Dixon in response to questions about the change in design of the Trad D whistle head. See link to photo of my Dixon head collection.
https://drive.google.com/open?id=13Lz5ot7OufzLZ4DLaj-kdxhziLD5tr9SLeft to right:
Head 1 - Dixon Trad D (Brass ) purchased maybe 2010-11. I call this Old Trad D.
Head 2 - Dixon Trad D (Nickel) purchased December 2013. I call this New Trad D.
Head 3 - One of the two heads Tony sent me.
Head 4- Another of the two Tony sent me.
Head 5 - A Dixon DX001 polymer, which I just bought recently.
Without an micrometer to provide measurements, the test heads 3 & 4, seem identical.
My Dixon DX001 polymer head area seems identical to the 3 &4 heads , as if they were cut off from the DX001 body. Even the Dixon logo seems the same size and location. DX001 plays similarly to the the Old Trad D #1 in my opinion.
Neither of the test heads sent, 3 &4, fit properly on the body of either Trad D. I can force one of those onto the Trad D body but it is very hard to get on and extremely hard to get off. I have good hand strength and had to resort to a technique Bill Ochs once used to remove whistle heads.
The head of the Old Trad D [#1] that I prefer appears to be similar in dimensions to the heads sent, although the logo is placed differently.
To allow the sent heads to fit, I sanded (with #400 paper) the top of the tube of my nickel Trad D and also slightly the inside of the #4 head sent me. It still was a snug fit but manageable.
My Nickel body using the #4 head Tony sent now sounds similar to my old brass Trad D, which was my preferred whistle. These are all good whistles. Note that on Chiff and Fipple a member called "Sedi" compared his Trad D Nickel to his DX001 and arrived at the same opinion that I did.
Hope this makes sense.