As you can see, the "horseshoe" washer sits directly under the hexagon nut in the shallow recess of the aluminium disc. This disc sits on two pins. You need only back the nut off enough to get out the horseshoe washer. Don't back off any more once resistance is felt, because it will "self- withdraw" the A.T.D. from the magneto shaft. If you haven't got a horseshoe washer you can make one. A good quality FLAT washer of half inch i.d. for memory about 1/16" thick and o.d. to suit the recess in the aluminium disc.Cut or mill the half-inch slot. Chances are it will be better than the original part as these were always soft, and deformed in service. Persevere!!!