Transfer Case Front Yoke Seal Washer

I replaced the front output shaft seal without consulting my manual. I know I did not put back a washer behind the yoke nut (don't remember one coming off), and now it's leaking worse than it was before. After much diligent research (cracked open the manual), I realize the seal washer may play an important role. Does anyone know the specs on this washer, is it rubber or metal?

Stuart