I have seen them at MB dealers. There are two different types. One for early rims up to 1985, and the other for 1986-1991 wheels. I think they are around $20 each.

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1994 E320 94k
1991 300TE 105K
1985 SD 184k
1983 SD 167K
1970 280SE 54K, sold
1981 SD 271k, sold
1984 300D 145K, sold