This front steering knuckle o-ring fits:

1986 & 1987 TRX350 / TRX 350 Fourtrax

1987 thru 1989 TRX350D / TRX350 D / TRX 350 D Fourtrax Foreman

This genuine part has been discontinued and is no longer available from Honda 

If you rebuild or restore any type of vehicle you know how important it is to use genuine / original factory / or high quality aftermarket parts. A repair or restoration project is intended to return that machine to as close to original as possible, all the way down to the front steering knuckle o-ring.

If there's even a chance you'll need this/these replacement part(s) in the future you'd better get them NOW!. Keep this in mind,  replacement parts are always good to have. When you decide to sell your Honda, including a few extra replacement parts always sweetens the deal for both buyer and seller.