If you can remove the lock nut without the inner screw rotating too much and messing up the aerobic chambers, you could cut a new screw slot in the screw with a Dremel and a thin disc. Might not be able to use a regular screwdriver in it, but a small narrow one shoud still work in the more "submerged" slot.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)