You have the early type steering wheel with horn ring levers protruding the arms of the steering wheel. You need the earlier plastic plug, PN A 111 464 00 24, 3 pcs. They are available at Mercedes, but are expensive, $50 a piece. It is the one with two sets of ribs on its ends.
I do not know if the newer version would do the job and fit.
The PN for the newer one is A 115 464 00 35 and they are $7 a piece at Mercedes.