Vifa / Peerless OC25SC65-04 25mm Dome Tweeter Capsule.
This tweeter features a 25 mm 4 ohm voice coil, a damped fabric dome giving a smooth frequency response and ferrofluid in the gap of the neodymium magnet for enhanced cooling of the voice coil. Twist-lock bayonet mounting for easy installation into custom faceplates / baffles. A full datasheet can be downloaded from the bottom of this page.
Excellent replacement for B&W P4, P5, P6, CDM1, Mission M7, M71, M72, M73, M74 and many others.