Vifa / Peerless XT19TD00-04 19mm Ring Radiator Tweeter, 4Ω.
Patented ring radiator diaphragm and central waveguide / phase plug give a dynamic extended response to 30Khz with good dispersion. This tweeter also features a tuned rear chamber giving a low Fs and extended low frequency response. The specially profiled 94mm diameter faceplate features recessed mounting holes for minimal reflection.
A full datasheet can be downloaded from the bottom of this page.