Some measurements on some tested tweeters:
1) Coral HE98, 1" Silk Dome Tweeter, loaded in Trasmission Line
Impedance Response (note the anomaly, casused -I think- by the Trasmission Line):

Frequency Response:

2) Ciare PT383, a 1.5" Dome tweeter that, in my opinion, is the "T38 Wave Guide", mounted on Chario Loudspeakers Costellation serie (www.chario.it)
Some Photos:



Impedance Response:

Frequency Response (gated):

Look at the "official" frequency response, provided by Ciare:

Now Look at some measurements on Chario Costellation Ursa Major loudspeaker:

It seems that Chario applied a 12dB HP Filter on a tweeter like the one I have... And what abuot the "official" Ciare response? What tweeter is that??
Think about it, people...