the pin on the older tumblers is actually on the back of the assembly. i think it can be accessed if you drop the knee panel and reach up inside. if not you may have to remove the cluster.

edit: i've never actually done this procedure. just researched it because i thought i had to replace the tumbler on a 77 240D. the old tumblers i believe were used up to 1980. if you have to replace it they are not compatible with the newer version, which is readily available for very cheap.

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1981 NA 300D 310k miles