Vifa / Peerless BC25SC06-04 25mm horn loaded dome tweeter.
This high efficiency tweeter features a 4 ohm 25 mm voice coil, high energy Neodymium magnet assembly, a damped textile dome and ferrofluid in the magnetic gap for enhanced cooling of the voice coil. The BC25SC06-04 has a compact horn loaded faceplate (70mm diameter) with recessed mounting holes, and comes complete with a foam sealing gasket to reduce vibration. A finned heatsink aids cooling.
A full datasheet can be downloaded from the bottom of this page.