And you can't tell a TK that it's and NX, or vice-versa. Sounds like you already sent it back, so it's moot, but you would read the radio and go by what that says. Yes, some earlier models would let you tell one thing it's something else. I have a TK-370G that thinks it's a TK372E, which let me put in 440-450 freqs. But you can't do that kind of thing with D1.
Yes, it is possible for someone to take a TK guts and put it on a NX plate, and forget to change the model/SN tag. But the *radio information* within D1 would still report a TK with the original SN. I think Kenwood can probably change it, but those tools aren't out in the wild.
Oh, and the firmware is the same for both. The TK's were discontinued a while ago, so that's probably why it isn't listed.