Vifa / Peerless NE25VTS-04, 25mm Dome Tweeter, Neo magnet, no ferrofluid.
This 25mm 4 ohm tweeter features leading-edge transducer technology packaged in a cutting edge, small form factor housing. Features include a Neodymium-Iron-Boron magnet (NdFeB) and a copper capped pole piece for an extended frequency response with reduced distortion. The aluminium rear chamber offers extended low frequency performance whilst doubling as a heat sink. The silk dome profile is designed for optimal sound quality and dispersion. The faceplate is aluminium with a plastic grille for protection of the tweeter diaphragm.
A full datasheet can be downloaded from the bottom of this page.