This is the stock white brake fluid reservoir cap. If yours has been damaged or cracked, this is a new OEM part. The actual outer plastic white piece is sold separate from the inner black rubber diaphragm, so you can order the parts separately or add to cart as a set.
The rubber diaphragm simply slips/pops into the white cap.
This fits the 3000GT/GTO/Stealth as well as several other Mitsubishis.