The conical washers are not a substitute for a thrust race. The thrust forces in the Mardave diff are all taken through the main (radial) bearing, as is common in most pan cars. It is a reason they fail so often because these bearings aren't designed to take large thrust loads.
The conical washers allow a degree of modulation to be had between loose and fully locked. The more washers you use, the more turns of the nut you will have in the adjustment range. Some companies use a stiff compression spring instead but the effect is the same.
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