Montesa 4RT Twin Air Filter. 

Features:
* washable and reusable - reusing a Twin Air filter 30 times is common
* dual-layer foam made to Twin Air specifications, with cells that are evenly sized and distributed
* flame-retardant foam layer laminated to the filter eliminates the restrictive, stock backfire screen
* minimized seams made possible by a revolutionary micro-gluing process
* greaseless sealing ring
* assembled by hand and inspected for a perfect fit
* packaged in a resealable and reusable bag