The brass washer is standard MEC equipment when you're using their fixed bars and ball powders. Instructions are on the back page of the 1983 manual.
Magnum high antimony shot will meter light, just as a hard cast bullet weighs less than a pure lead or 40:1 lead-tin bullet will, when they're cast from the same mould. Your pellet count doesn't suffer near as much. IIRC, magnum shot runs 6-8% antimony.